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Metric prefix
View on WikipediaA metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic. Each prefix has a unique symbol that is prepended to any unit symbol. The prefix kilo, for example, may be added to gram to indicate multiplication by one thousand: one kilogram is equal to one thousand grams. The prefix milli, likewise, may be added to metre to indicate division by one thousand, so one millimetre is equal to one thousandth of a metre.
Decimal multiplicative prefixes have been a feature of all forms of the metric system, with six of these dating back to the system's introduction in the 1790s. Metric prefixes have also been used with some non-metric units. The SI prefixes are metric prefixes that were standardised for use in the International System of Units (SI) by the International Bureau of Weights and Measures (BIPM) in resolutions dating from 1960 to 2022.[1][2] Since 2009, they have formed part of the ISO/IEC 80000 standard. They are also used in the Unified Code for Units of Measure (UCUM).
List of SI prefixes
[edit]The BIPM specifies twenty-four prefixes for the International System of Units (SI).
| Prefix | Base 10 | Decimal | Adoption [nb 1] | |
|---|---|---|---|---|
| Name | Symbol | |||
| quetta | Q | 1030 | 1000000000000000000000000000000 | 2022[3] |
| ronna | R | 1027 | 1000000000000000000000000000 | |
| yotta | Y | 1024 | 1000000000000000000000000 | 1991 |
| zetta | Z | 1021 | 1000000000000000000000 | |
| exa | E | 1018 | 1000000000000000000 | 1975[4] |
| peta | P | 1015 | 1000000000000000 | |
| tera | T | 1012 | 1000000000000 | 1960 |
| giga | G | 109 | 1000000000 | |
| mega | M | 106 | 1000000 | 1873 |
| kilo | k | 103 | 1000 | 1795 |
| hecto | h | 102 | 100 | |
| deca | da | 101 | 10 | |
| — | — | 100 | 1 | — |
| deci | d | 10−1 | 0.1 | 1795 |
| centi | c | 10−2 | 0.01 | |
| milli | m | 10−3 | 0.001 | |
| micro | μ | 10−6 | 0.000001 | 1873 |
| nano | n | 10−9 | 0.000000001 | 1960 |
| pico | p | 10−12 | 0.000000000001 | |
| femto | f | 10−15 | 0.000000000000001 | 1964 |
| atto | a | 10−18 | 0.000000000000000001 | |
| zepto | z | 10−21 | 0.000000000000000000001 | 1991 |
| yocto | y | 10−24 | 0.000000000000000000000001 | |
| ronto | r | 10−27 | 0.000000000000000000000000001 | 2022[3] |
| quecto | q | 10−30 | 0.000000000000000000000000000001 | |
| ||||
The first uses of prefixes in SI date back to the definition of kilogram after the French Revolution at the end of the 18th century. Several more prefixes came into use, and were recognised by the 1947 IUPAC 14th International Conference of Chemistry[5] before being officially adopted for the first time in 1960.[6]
The prefixes that were most recently adopted are ronna, quetta, ronto, and quecto. These prefixes were adopted in 2022, after a proposal from British metrologist Richard J. C. Brown. (Before 2022, Q/q and R/r were the only Latin letters available for abbreviations, with all other Latin letters being already used for other prefixes (a, c, d, E, f, G, h, k, M, m, n, P, p, T, Y, y, Z, z) or already used for SI units (including: SI base units, SI derived units, Non-SI units mentioned in the SI) (A, B, C, d, F, g, H, h, J, K, L, m, N, S, s, T, t, u, V, W) or easily confused with mathematical operators (I and l are easily confused with 1, O and o are easily confused with 0, X and x are easily confused with ×).) The large prefixes ronna and quetta were adopted in anticipation of needs for use in data science, and because unofficial prefixes that did not meet SI requirements were already circulating. The small prefixes were also added, even without such a driver, in order to maintain symmetry.[7]
The prefixes from peta to quetta are based on the Ancient Greek or Ancient Latin numbers from 5 to 10, referring to the 5th through 10th powers of 103. The initial letter h has been removed from some of these stems and the initial letters z, y, r, and q have been added, ascending in reverse alphabetical order, to avoid confusion with other metric prefixes.
Rules
[edit]- The symbols for the units of measure are combined with the symbols for each prefix name. The SI symbols for kilometre, kilogram, and kilowatt, for instance, are km, kg, and kW, respectively. (The symbol for kilo is k.) Except for the early prefixes of kilo, hecto, and deca, the symbols for the prefixes for multiples are uppercase letters, and those for the prefixes for submultiples are lowercase letters.[8]
- All of the metric prefix symbols are made from upper- and lower-case Latin letters except for the symbol for micro, which is uniquely a Greek letter μ.[a]
- The prefix symbols are always prepended to the symbol for the unit without any intervening space or punctuation.[9] This distinguishes a prefixed unit symbol from the product of unit symbols, for which a space or mid-height dot as separator is required. So, for instance, while 'ms' means millisecond, 'm s' or 'm·s' means metre-second.
- Prefixes corresponding to an integer power of one thousand are generally preferred; the prefixes corresponding to tens (deci-, deca-) and hundreds (centi-, hecto-) are less common and are disfavoured in certain fields. Hence, 100 m is preferred over 1 hm (hectometre) or 10 dam (decametres). The prefixes deci- and centi-, and less frequently hecto and deca, are generally used for informal purposes; the centimetre (cm) is especially common. Some modern building codes require that the millimetre be used in preference to the centimetre, because "use of centimetres leads to extensive usage of decimal points and confusion".[10] These prefixes are also commonly used to create metric units corresponding to older conventional units, for example hectares and hectopascals.
- Prefixes may not be used in combination on a single symbol. This includes the case of the base unit kilogram, which already contains a prefix. For example, milligram (mg) is used instead of microkilogram (μkg).
- During mathematical operations, prefixes are treated as multiplicative factors. For example, 5 km is treated as 5000 m, which allows all quantities based on the same unit to be factored together even if they have different prefixes.
- A prefix symbol attached to a unit symbol is included when the unit is raised to a power. For example, 1 km2 denotes 1 km × 1 km = 106 m2, not 103 m2.
Usage
[edit]Examples
[edit]- The mass of an electron is about 1 rg (rontogram).[7][b]
- The mass of 1 litre of water is about 1 kg (kilogram).[12]
- The mass of the Earth is about 6 Rg (ronnagrams).[7]
- The mass of Jupiter is about 2 Qg (quettagrams).[7]
Examples of powers of units with metric prefixes
[edit]- 1 km2 means one square kilometre, or the area of a square of 1000 m by 1000 m. In other words, an area of 1000000 square metres and not 1000 square metres.
- 2 Mm3 means two cubic megametres, or the volume of two cubes of 1000000 m by 1000000 m by 1000000 m, i.e. 2×1018 m3, and not 2000000 cubic metres (2×106 m3).
Examples with prefixes and powers
[edit]- 5 mV × 5 mA = 5×10−3 V × 5×10−3 A = 25×10−6 V⋅A = 25 μW.
- 5.00 mV + 10 μV = 5.00 mV + 0.01 mV = 5.01 mV.
- 5 cm = 5×10−2 m = 5 × 0.01 m = 0.05 m.
- 9 km2 = 9 × (103 m)2 = 9 × (103)2 × m2 = 9×106 m2 = 9 × 1000000 m2 = 9000000 m2.
- 3 MW = 3×106 W = 3 × 1000000 W = 3000000 W.
Micro symbol
[edit]When mega and micro were adopted in 1873, three prefixes existed starting with "m". It was necessary to use a symbol other than upper and lowercase 'm'. Eventually the Greek letter "μ" was adopted.
With the lack of a "μ" key on most typewriters, as well as computer keyboards, various other abbreviations remained common, including "mc", "mic", M, and "u".
From about 1960 onwards, "u" prevailed in type-written documents.[c] Because ASCII, EBCDIC, and other common encodings lacked code-points for "μ", this tradition remained even as computers replaced typewriters.
When ISO 8859-1 was created, it included the "μ" symbol for micro at codepoint 0xB5; later, the whole of ISO 8859-1 was incorporated into the initial version of Unicode. Many fonts that support both characters render them identically, but because the micro sign and the Greek lower-case letter have different applications (normally, a Greek letter would be used with other Greek letters, but the micro sign is never used like that), some fonts render them differently, e.g. Linux Libertine and Segoe UI.[citation needed]
Keyboard entry
[edit]Most English-language keyboards do not have a "μ" key, so it is necessary to use a key-code; this varies depending on the operating system, physical keyboard layout, and user's language.
- For all keyboard layouts
-
- On Microsoft Windows systems,
- arbitrary Unicode codepoints can be entered in decimal with: Alt sustained, 0 1 8 1, and releasing Alt. A leading "0" is required
(this registers as the corresponding Unicode hexadecimal code-point, 0xB5 = 181.), or - arbitrary Unicode codepoints can be entered in hexadecimal as: Alt++b5
(up to 5 hexadecimal characters, not counting the leading '+', upper or lower case), or - in the tradition of MS-DOS, IBM code page 437 one can also enter old code-points in decimal: Alt+230
(the leading zero must be omitted);
- arbitrary Unicode codepoints can be entered in decimal with: Alt sustained, 0 1 8 1, and releasing Alt. A leading "0" is required
- On Linux systems,
- under X11, when a Compose key has been enabled: Composemu
- under X11, with ibus version 1.5.19 (or higher) active, and a non-composing input method selected: The default keybinding for starting codepoint input is Ctrl+⇧ Shift+u. The key sequence Ctrl+⇧ Shift+u b 5 space then produces U+00B5, the micro sign.
- on the VGA console's virtual terminals like tty1: arbitrary Unicode codepoints can be entered in decimal as: Alt sustained, 1 8 1, and releasing Alt. A leading "0" is not required.
- On Microsoft Windows systems,
- For QWERTY keyboard layouts
Typesetting in LaTeX
[edit]The LaTeX typesetting system features an SIunitx package in which the units of measurement are spelled out, for example,
\qty{3}{\tera\hertz} formats as "3 THz".[13]
Application to units of measurement
[edit]The use of prefixes can be traced back to the introduction of the metric system in the 1790s, long before the 1960 introduction of the SI.[14] The prefixes, including those introduced after 1960, are used with any metric unit, whether officially included in the SI or not (e.g., millidyne and milligauss). Metric prefixes may also be used with some non-metric units, but not, for example, with the non-SI units of time.[15]
Metric units
[edit]Mass
[edit]The units kilogram, gram, milligram, microgram, and smaller are commonly used for measurement of mass. However, megagram, gigagram, and larger are rarely used; tonnes (and kilotonnes, megatonnes, etc.) or scientific notation are used instead. The megagram does not share the risk of confusion that the tonne has with other units with the name "ton".
The kilogram is the only coherent unit of the International System of Units that includes a metric prefix.[16]: 144
Volume
[edit]The litre (equal to a cubic decimetre), millilitre (equal to a cubic centimetre), microlitre, and smaller are common. In Europe, the centilitre is often used for liquids (e.g. bottles or servings of wine), and the decilitre is used less frequently. Bulk agricultural products, such as grain, beer and wine, often use the hectolitre (100 litres).[citation needed]
Larger volumes are usually denoted in kilolitres, megalitres or gigalitres, or else in cubic metres (1 cubic metre = 1 kilolitre) or cubic kilometres (1 cubic kilometre = 1 teralitre). For scientific (other than medical) purposes, the SI unit of cubic metre is usually used, with scientific notation rather than prefixes.[citation needed]
Length
[edit]The kilometre, metre, centimetre, millimetre, and smaller units are common. The decimetre is rarely used. The micrometre is often referred to by the older non-SI name micron, which is officially deprecated. In some fields, such as chemistry, the ångström (0.1 nm) has been used commonly instead of the nanometre. The femtometre, used mainly in particle physics, is sometimes called a fermi. For large scales, megametre, gigametre, and larger are rarely used. Instead, ad hoc non-metric units are used, such as the solar radius, astronomical units, light years, and parsecs, and less commonly large multiples (e.g. millions) of kilometres; the astronomical unit is mentioned in the SI standards as an accepted non-SI unit.[17]
Time
[edit]Prefixes for the SI standard unit second are most commonly encountered for quantities less than one second. For larger quantities, the system of minutes (60 seconds), hours (60 minutes) and days (24 hours) is accepted for use with the SI and more commonly used. When speaking of spans of time, the length of the day is usually standardised to 86400 seconds so as not to create issues with the irregular leap second.[citation needed]
Larger multiples of the second such as kiloseconds and megaseconds are occasionally encountered in scientific contexts, but are seldom used in common parlance. For long-scale scientific work, particularly in astronomy, the Julian year or annum (a) is a standardised variant of the year, equal to exactly 31557600 seconds (365+ 1 /4 days). The unit is so named because it was the average length of a year in the Julian calendar. Long time periods are then expressed by using metric prefixes with the annum, such as megaannum (Ma) or gigaannum (Ga).[citation needed]
Angle
[edit]The SI unit of angle is the radian, but degrees, as well as arc-minutes and arc-seconds, see some scientific use in fields such as astronomy.[18]
Temperature
[edit]Common practice does not typically use the flexibility allowed by official policy in the case of the degree Celsius (°C). NIST states:[19] "Prefix symbols may be used with the unit symbol °C and prefix names may be used with the unit name degree Celsius. For example, 12 m°C (12 millidegrees Celsius) is acceptable." In practice, it is more common for prefixes to be used with the kelvin when it is desirable to denote extremely large or small absolute temperatures or temperature differences. Thus, temperatures of star interiors may be given with the unit of MK (megakelvin), and molecular cooling may be given with the unit mK (millikelvin).[citation needed]
Energy
[edit]In use the joule and kilojoule are common, with larger multiples seen in limited contexts. In addition, the kilowatt-hour, a composite unit formed from the kilowatt and hour, is often used for electrical energy; other multiples can be formed by modifying the prefix of watt (e.g. terawatt-hour).[citation needed]
Several definitions exist for the non-SI unit calorie. Distinguished are gram calories and kilogram calories. One kilogram calorie, which equals one thousand gram calories, often appears capitalized and without a prefix (i.e. Cal) when referring to "dietary calories" in food.[20] It is common to apply metric prefixes to the gram calorie, but not to the kilogram calorie: thus, 1 kcal = 1000 cal = 1 Cal.
Non-metric units
[edit]Metric prefixes are widely used outside the metric SI system. Common examples include the megabyte and the decibel. Metric prefixes rarely appear with imperial or US units except in some special cases (e.g., microinch, kilofoot, kilopound). They are also used with other specialised units used in particular fields (e.g., megaelectronvolt, gigaparsec, millibarn, kilodalton). In astronomy, geology, and palaeontology, the year, with symbol 'a' (from the Latin annus), is commonly used with metric prefixes: ka, Ma, and Ga.[21]
Official policies about the use of SI prefixes with non-SI units vary slightly between the International Bureau of Weights and Measures (BIPM) and the American National Institute of Standards and Technology (NIST). For instance, the NIST advises that "to avoid confusion, prefix symbols (and prefix names) are not used with the time-related unit symbols (names) min (minute), h (hour), d (day); nor with the angle-related symbols (names) ° (degree), ' (minute), and ″ (second),[19] whereas the BIPM adds information about the use of prefixes with the symbol as for arcsecond when they state: "However astronomers use milliarcsecond, which they denote mas, and microarcsecond, μas, which they use as units for measuring very small angles."[22]
Non-standard prefixes
[edit]
Obsolete metric prefixes
[edit]Some of the prefixes formerly used in the metric system have fallen into disuse and were not adopted into the SI.[23][24][25][page needed] The decimal prefix for ten thousand, myria- (sometimes spelt myrio-), and the early binary prefixes double- (2×) and demi- (1/2×) were parts of the original metric system adopted by France in 1795,[26][d] but were not retained when the SI prefixes were internationally adopted by the 11th CGPM conference in 1960.
Other metric prefixes used historically include hebdo- (107) and micri- (10−14).
Double prefixes
[edit]Double prefixes have been used in the past, such as micromillimetres or millimicrons (now nanometres), micromicrofarads (μμF; now picofarads, pF), kilomegatonnes (now gigatonnes), hectokilometres (now 100 kilometres) and the derived adjective hectokilometric (typically used for qualifying the fuel consumption measures).[27] These are not compatible with the SI.
Other obsolete double prefixes included "decimilli-" (10−4), which was contracted to "dimi-"[28] and standardised in France up to 1961.
There are no more letters of the Latin alphabet available for new prefixes (all the unused letters are already used for units). As such, Richard J. C. Brown (who proposed the prefixes adopted for 10±27 and 10±30) has proposed a reintroduction of compound prefixes (e.g. kiloquetta- for 1033) if a driver for prefixes at such scales ever materialises, with a restriction that the last prefix must always be quetta- or quecto-. This usage has not been approved by the BIPM.[29][30]
Similar symbols and abbreviations
[edit]In written English, the symbol K is often used informally to indicate a multiple of thousand in many contexts. For example, one may talk of a 40K salary (40000), or call the Year 2000 problem the Y2K problem. In these cases, an uppercase K is often used with an implied unit (although it could then be confused with the symbol for the kelvin temperature unit if the context is unclear). This informal postfix is read or spoken as "thousand", "grand", or just "k".
The financial and general news media mostly use m or M, b or B, and t or T as abbreviations for million, billion (109) and trillion (1012), respectively, for large quantities, typically currency[31] and population.[32]
The medical and automotive fields in the United States use the abbreviations cc or ccm for cubic centimetres. One cubic centimetre is equal to one millilitre.
For nearly a century,[vague] engineers used the abbreviation MCM to designate a "thousand circular mils" in specifying the cross-sectional area of large electrical cables. Since the mid-1990s, kcmil has been adopted as the official designation of a thousand circular mils, but the designation MCM still remains in wide use. A similar system is used in natural gas sales in the United States: m (or M) for thousands and mm (or MM) for millions (thousand thousands) of British thermal units or therms, and in the oil industry,[33] where MMbbl is the symbol for "millions of barrels". These usages of the capital letter M for "thousand" in MCM is from Roman numerals, in which M means 1000.[34][33]
See also
[edit]- Binary prefix – Prefix indicating a power of two
- CJK Compatibility – Unicode block including precomposed unit symbols with prefixes, such as U+3371 ㍱ SQUARE HPA
- E1 series (preferred numbers) – Series of preferred values for passive electrical components
- Engineering notation – Version of scientific notation where the exponent of ten reflects powers of 1000
- Indian numbering system – Indian convention of naming large numbers
- International vocabulary of metrology
- ISO/IEC 80000 – International standard on physical quantities and units of measurement
- Numeral prefix – Prefix derived from numerals or other numbers
- Order of magnitude – Scale of numbers with a fixed ratio
- Orders of magnitude (data) – Computer data measurements and scales
- RKM code – Notation to specify resistor and capacitor values
- Unified Code for Units of Measure – System of codes for unambiguously representing measurement units
Footnotes
[edit]- ^ For ASCII compatibility in general text usage, μ is frequently substituted with the Latin letter u.
- ^ me = 9.1093837139(28)×10−31 kg[11]. Converting to grams gives 9.1093837015×10−28 g. Rounding to the nearest power of ten gives 1×10−27 g, or 1 rg.
- ^ Sometimes the symbol 'u' is marked by adding a downstroke using a pen or pencil, or a slash '/u'.
- ^ "Art. 8. Dans les poids et mesures de capacité, chacune des mesures décimales de ces deux genres aura son double et sa moitié, afin de donner à la vente des divers objets toute la commodité que l'on peut désirer. Il y aura donc le double-litre et le demi-litre, le double-hectogramme et le demi-hectogramme, et ainsi des autres." ("Art. 8. In the weights and measures of capacity, each of the decimal measures of these two kinds will have its double and its half, in order to give to the sale of the various articles all the convenience that one can desire. There will therefore be the double-litre and the half-litre, the double-hectogram and the half-hectogram, and so on.")[26]
References
[edit]- ^ "SI prefixes". Bipm.org. Bureau International des Poids et Mesures. Archived from the original on 2014-09-12. Retrieved 2023-06-13.
- ^ "Resolutions of the General Conference on Weights and Measures (27th meeting)" (PDF) (in French and English). Bureau International des Poids et Mesures. 2022-11-18. Archived from the original on 2023-06-13. Retrieved 2024-05-18.
- ^ a b On the extension of the range of SI prefixes. CGPM. Resolution 3 of the 27th CGPM (2022). BIPM. 2022-11-18. doi:10.59161/cgpm2022res3e. Retrieved 2023-02-05.
- ^ "Metric (SI) Prefixes". NIST.
- ^ Comptes rendus de la Quatorzième Conférence (in French). London: IUPAC. 1947-07-24.
- ^ "Resolution 12 of the 11th meeting of the CGPM". Bipm.org. Bureau International des Poids et Mesures. 1960. Archived from the original on 2013-02-13. Retrieved 2023-09-12.
- ^ a b c d Sample, Ian (2022-11-18), "Earth weighs in at six ronnagrams as new prefixes picked for big and small", The Guardian, retrieved 2022-12-14
- ^ "Metric Prefixes and SI Units". learn.sparkfun.com. tutorials. Retrieved 2020-01-26.
- ^ "SI Unit rules and style conventions checklist". nist.gov. NIST. September 2004 [February 1998].
- ^ Metric Design Guide (PDF) (Report). Public Buildings Service. U.S. General Services Administration. September 1995. PBS-PQ260. Archived from the original (PDF) on 2011-12-15. Retrieved 2018-04-21 – via National Institute of Building Sciences.
- ^ "2022 CODATA Value: electron mass". The NIST Reference on Constants, Units, and Uncertainty. NIST. May 2024. Retrieved 2024-05-18.
- ^ "Kilogram | mass, weight, SI unit | Britannica". 2024-03-15.
- ^ Kottwitz, Stefan (2015-10-28), LaTeX Cookbook, Packt Publishing Ltd, pp. 158–9, ISBN 978-1-78439-630-5
- ^ "Metric (SI) Prefixes". NIST. 2010-01-13.
- ^ The International System of Units (PDF), V3.01 (9th ed.), International Bureau of Weights and Measures, Aug 2024, p. 145, ISBN 978-92-822-2272-0
- ^ The International System of Units (PDF), V3.01 (9th ed.), International Bureau of Weights and Measures, Aug 2024, ISBN 978-92-822-2272-0
- ^ "The International System of Units (SI)". International Bureau of Weights and Measures. Retrieved 2025-06-28.
- ^ Barbieri, C., & Bertini, I. (2020). Fundamentals of Astronomy (2nd ed.), p. 1-2. CRC Press. https://doi.org/10.1201/9780429287305
- ^ a b Thompson, Ambler; Taylor, Barry N. (March 2008). Special Publication 811 (Report) (2008 ed.). National Institute of Standards and Technology. Retrieved 2018-06-21 – via nist.gov.
- ^ Conn, Carole; Kravitz, Len. "Remarkable Calorie". University of New Mexico. Retrieved 2017-05-22.
- ^ Gargaud, Muriel; Amils, Ricardo; Cleaves, Henderson James (26 May 2011). "Ga". Encyclopedia of Astrobiology. Springer Science & Business Media. p. 621. ISBN 978-3-642-11271-3.
- ^ The International System of Units (SI) (Report). SI Brochure. International Bureau of Weights and Measures. Retrieved 2017-03-05.
- ^ "H.R. 596, An Act to authorize the use of the metric system of weights and measures". 29th Congress of the United States, Session 1. 1866-05-13. Archived from the original on 2015-07-05.
- ^ Brewster, David (1830). The Edinburgh Encyclopædia. Vol. 12. Edinburgh, UK: William Blackwood, John Waugh, John Murray, Baldwin & Cradock, J. M. Richardson. p. 494. Retrieved 2015-10-09.
- ^ Brewster, David (1832). The Edinburgh Encyclopaedia. Vol. 12 (1st American ed.). Joseph and Edward Parker. Retrieved 2015-10-09.
- ^ a b "La loi du 18 Germinal an 3". L'histoire du mètre [The History of the Metre] (in French). Archived from the original on 2022-11-26. Retrieved 2015-10-12 – via histoire.du.metre.free.fr.
Décision de tracer le mètre, unité fondamentale, sur une règle de platine. Nomenclature des « mesures républicaines ». Reprise de la triangulation
[The Law of 18 Germinal [month], Year 3: Decision to draw the fundamental unit metre on a platinum ruler. Nomenclature of "republican measures". Resumption of the triangulation] - ^ Rowlett, Russ (2008) [2000]. "millimicro-". How Many? A dictionary of units of measurement. University of North Carolina at Chapel Hill. Archived from the original on 2016-08-29. Retrieved 2016-08-29.
- ^ Danloux-Dumesnils, Maurice (1969). The Metric System: A critical study of its principles and practice. The Athlone Press. p. 34. ISBN 9780485120134. Retrieved 2015-10-09. (a translation of the French original Esprit et bon usage du système métrique, 1965 )
- ^ Brown, Richard J. C. (2022-04-27). "Reply to 'Facing a shortage of the Latin letters for the prospective new SI symbols: Alternative proposal for the new SI prefixes'". Accreditation and Quality Assurance. 27 (3): 143–144. doi:10.1007/s00769-022-01499-7. S2CID 248397680.
- ^ Brown, Richard J.;C. (2019). "Considerations on compound SI prefixes". Measurement. 140: 237–239. Bibcode:2019Meas..140..237B. doi:10.1016/j.measurement.2019.04.024. S2CID 146092009.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ "Obama unveils $3.8T budget proposal". Canadian Broadcasting Corporation. Associated Press. 2012-02-13. Retrieved 2012-03-01.
- ^ "More than 65M Flock to Discovery's Planet Earth". Multichannel.com. Retrieved 2012-03-01.
- ^ a b Purcell, P. (2007). "Disambiguating M". PESA News. Archived from the original on 2012-03-25. Retrieved 2012-03-01.
- ^ "What Is The Difference Between MCM and Kcmil?". Reference. 2015-08-04. Retrieved 2016-09-05.
External links
[edit]- International Bureau of Weights and Measures (BIPM)
- SI prefixes at BIPM
- US NIST Definitions of the SI units: The twenty SI prefixes
- US NIST Definitions of the SI units: The binary prefixes
Metric prefix
View on GrokipediaHistory and Development
Origins in the Metric System
The metric system emerged in late 18th-century France amid the revolutionary fervor following the French Revolution of 1789, as reformers sought to establish a rational, decimal-based framework for measurements that would unify the nation and transcend local variations. Prior to this, France employed over 700 disparate units influenced by regional customs, human anatomy, and feudal traditions, leading to inconsistencies in trade, science, and administration. The National Assembly, responding to widespread demands for standardization, commissioned scientists in 1790 to devise a universal system grounded in natural invariants, such as fractions of the Earth's meridian, to ensure invariance and accessibility. This initiative reflected Enlightenment ideals of reason and equality, aiming to replace arbitrary seigneurial measures with a coherent, decimal structure applicable across disciplines.[5][6] On 7 April 1795, the French Academy of Sciences formally introduced the initial set of metric prefixes as part of the decimal metric system's legal adoption, enabling scalable multiples and submultiples of base units like the meter (for length) and kilogram (for mass) without redefining the fundamentals. The prefixes included myria- for 10⁴, kilo- for 10³, centi- for 10⁻², and milli- for 10⁻³, derived from Greek and Latin roots to denote powers of ten and facilitate everyday and scientific applications. These were among eight original prefixes—also encompassing deca-, hecto-, deci-, and myrio-—designed to create a harmonious progression from large to small scales, such as myriameter for vast distances or millimeter for precise dimensions. Key figures in this development included Jean-Charles de Borda, who chaired the 1791 commission defining the meter and advocated for decimal coherence, and Pierre Méchain, who, alongside Jean-Baptiste Delambre, conducted meridian measurements from 1792 to 1798 to calibrate the system empirically.[2][5] The introduction of these prefixes played a pivotal role in supplanting the patchwork of local units, such as the varying toises and livres across provinces, by allowing consistent decimal adjustments to base units for practical use in agriculture, commerce, and emerging industries. For instance, prefixes enabled the expression of a kilometer as 1,000 meters or a centigram as 0.01 gram, promoting interoperability without proliferation of new units. This foundational approach to scaling underscored the metric system's emphasis on simplicity and universality, laying the groundwork for its gradual international adoption despite initial resistance.[2][5]Standardization and Evolution
The International System of Units (SI) was formally established in 1960 by the 11th General Conference on Weights and Measures (CGPM), which adopted a coherent set of decimal prefixes to facilitate the expression of multiples and submultiples of SI units, including the base set such as deca-, hecto-, kilo-, deci-, centi-, milli-, and micro-, while adding mega-, giga-, tera-, nano-, and pico-.[2] This marked the transition from earlier ad hoc metric practices to a regulated international framework, with the CGPM obsoleting outdated prefixes like myria- and myrio- to streamline usage.[2] Subsequent expansions addressed growing scientific and technological demands. In 1964, the 12th CGPM introduced femto- (10^{-15}) and atto- (10^{-18}) for subatomic scales. The 15th CGPM in 1975 added peta- (10^{15}) and exa- (10^{18}) to accommodate large-scale measurements in fields like computing and energy. Further extension occurred in 1991 at the 19th CGPM, incorporating zetta- (10^{21}), zepto- (10^{-21}), yotta- (10^{24}), and yocto- (10^{-24}) to span an even broader range of magnitudes. The most recent update came in 2022 at the 27th CGPM, which added ronna- and ronto- (10^{27} and 10^{-27}) along with quetta- and quecto- (10^{30} and 10^{-30}), driven by needs in big data storage—such as quettabytes for exascale computing—and cosmological studies of sub-Planck scales.[7][8] This expansion brought the total to 24 prefixes, covering 10^{-30} to 10^{30} and ensuring decimal coherence across disciplines.[1] The International Bureau of Weights and Measures (BIPM) plays a central role in maintaining and updating the SI prefix list, recommending revisions to the CGPM based on metrological advancements and international consensus to preserve the system's uniformity and precision. Over time, this process has evolved from fragmented national conventions—rooted in 18th-century French origins—into a strictly regulated global standard.Standard SI Prefixes
Complete List of Prefixes
The International System of Units (SI) defines 24 prefixes for forming decimal multiples and submultiples of SI units, ranging from 10^{-30} to 10^{30}. These prefixes were established and expanded through resolutions of the General Conference on Weights and Measures (CGPM), with the most recent additions approved in 2022 to accommodate growing needs in scientific measurement, such as in data storage and particle physics.[1][7] The following table lists all approved SI prefixes in order from the smallest (submultiples) to the largest (multiples), including their names, symbols, powers of 10, and years of formal adoption where specified by the International Bureau of Weights and Measures (BIPM). Years reflect CGPM resolutions; earlier prefixes like deci and centi date to the original metric system's adoption in 1795 but were standardized in the SI in 1960.[1][4]| Prefix Name | Symbol | Power of 10 | Year of Adoption |
|---|---|---|---|
| quecto | q | 10^{-30} | 2022 |
| ronto | r | 10^{-27} | 2022 |
| yocto | y | 10^{-24} | 1991 |
| zepto | z | 10^{-21} | 1991 |
| atto | a | 10^{-18} | 1964 |
| femto | f | 10^{-15} | 1964 |
| pico | p | 10^{-12} | 1960 |
| nano | n | 10^{-9} | 1960 |
| micro | µ | 10^{-6} | 1960 |
| milli | m | 10^{-3} | 1960 |
| centi | c | 10^{-2} | 1960 |
| deci | d | 10^{-1} | 1960 |
| deca | da | 10^{1} | 1960 |
| hecto | h | 10^{2} | 1960 |
| kilo | k | 10^{3} | 1960 |
| mega | M | 10^{6} | 1960 |
| giga | G | 10^{9} | 1960 |
| tera | T | 10^{12} | 1960 |
| peta | P | 10^{15} | 1975 |
| exa | E | 10^{18} | 1975 |
| zetta | Z | 10^{21} | 1991 |
| yotta | Y | 10^{24} | 1991 |
| ronna | R | 10^{27} | 2022 |
| quetta | Q | 10^{30} | 2022 |
Naming Conventions and Symbols
The names of SI prefixes are derived primarily from Greek and Latin roots, as well as other languages, denoting numerical values, sizes, or concepts. For example, mega- derives from the Greek mégas meaning "great" for 10^6, while deci- comes from the Latin decem meaning "ten" for 10^{-1}.[2][9] More recent prefixes, particularly those introduced in 2022 to extend the range for extreme scales in data and measurements, shift toward numerical derivations for consistency and scalability; for instance, ronna- (10^{27}) combines elements from Greek ennéa ("nine") and Latin novem ("nine") to denote the ninth power of 10^3, while quetta- (10^{30}) derives from Latin decem ("ten") for the tenth power.[7][10] This approach ensures phonetic and orthographic harmony, with names ending in -a for multiples and -o for submultiples to maintain linguistic patterns established since the 1960s.[10] SI prefix symbols are formed as single characters, typically uppercase for multiples of 10^3 or higher (e.g., M for mega- at 10^6) and lowercase for submultiples (e.g., m for milli- at 10^{-3}), with exceptions like the lowercase k for kilo- (10^3) to align with early conventions.[10] Special cases include the Greek letter μ (mu) for micro- (10^{-6}), chosen for its phonetic fit and to avoid the Latin "u" already used elsewhere, ensuring symbols remain distinct and unambiguous.[2] Symbols for the 2022 additions follow this scheme, using uppercase R for ronna- and Q for quetta-, with lowercase r and q for their submultiple counterparts ronto- and quecto-.[7] To prevent ambiguity in scientific notation and unit combinations, SI prefix symbols must be unique, limited to one or two characters, and not overlap with base unit symbols or numerical digits (e.g., avoiding 'l' for litre confusion with '1' through separate styling rules).[10] The International Bureau of Weights and Measures (BIPM) enforces these guidelines via the General Conference on Weights and Measures (CGPM), selecting unused letters like Q and R for the 2022 extensions after evaluating global availability and avoiding conflicts with existing notations.[11] This systematic selection promotes international consistency, with all symbols rendered in upright roman type without spaces when attached to unit symbols, such as km for kilometer.[10] The 2022 CGPM updates specifically addressed scalability for emerging fields like big data and cosmology, extending prefixes to 10^{30} and 10^{-30} while preserving orthographic rules—names avoid double vowels or consonants for ease of pronunciation, and symbols prioritize rare letters to future-proof the system against further expansions.[7][12]Rules and Conventions
General Usage Rules
Metric prefixes, officially designated as SI prefixes, are used to indicate decimal multiples or submultiples of SI units by factors of , where is an integer ranging from -30 to +30.[1] These prefixes are attached to the names or symbols of base or derived SI units to form new units, such as the kilometer (km), which equals meters. A special case applies to the unit of mass: the kilogram (kg) is the only SI base unit whose name includes a prefix. Decimal multiples and submultiples of the kilogram are formed by attaching the appropriate prefix to the unit gram (g), for example, milligram (mg = kg).[10] They are not applied to dimensionless quantities or numbers alone, and no prefix exists or is used for the factor (unity).[10] In scientific and technical writing, SI prefixes are generally preferred over scientific notation for expressing quantities within typical scales, as they enhance readability and avoid ambiguity, except in cases of extremely large or small values beyond the prefix range.[13] Mixing prefixes with exponents on the same unit symbol is prohibited to maintain clarity and consistency (e.g., m, not km with an additional exponent).[10] This preference aligns with the goal of keeping numerical values between 0.1 and 1000 where possible, promoting coherent expression in the International System of Units (SI).[13] According to international standards, there is no space between a prefix symbol and the unit symbol it modifies, forming a single, inseparable entity (e.g., km, not k m). Similarly, when writing full names, the prefix and unit name are combined without a space or hyphen (e.g., millimeter).[10] These conventions are specified in ISO 80000-1 and the SI Brochure published by the International Bureau of Weights and Measures (BIPM), ensuring uniform application across global scientific communication.[10]Combining Prefixes with Powers
In the International System of Units (SI), when a prefixed unit is raised to a power, the exponent applies to the entire unit, including the prefix factor. For instance, the square kilometer is expressed as km², which mathematically equals (10³ m)² = 10⁶ m².[10] This convention ensures that the prefix's scaling factor is consistently multiplied by the power, avoiding ambiguity in derived units. Redundant notations, such as k m² to denote 10³ m², are discouraged because they separate the prefix from the unit symbol, violating the rule that prefixes form an inseparable part of the unit expression.[13] Standard SI practice prohibits the use of multiple prefixes or combining prefixes with additional powers in a single unit expression, as this leads to non-standard and confusing forms. For example, expressions like mμm (milli-micrometer) are not permitted; instead, a single prefix closest to unity, such as nm (nanometer) for 10⁻⁹ m, must be used. Similarly, for scales like 10⁶ m, the preferred form is Mm (megameter), rather than compounded forms like k km (kilo-kilometer). These restrictions maintain clarity and adherence to the decimal nature of SI prefixes, which strictly represent powers of 10.[10][13] In certain engineering and legacy contexts, double prefixes have occasionally appeared, such as kV/mm (kilovolt per millimeter) for electric field strength, but this is explicitly discouraged in favor of simplified equivalents like MV/m (megavolt per meter). Such usages stem from historical practices but do not conform to modern SI guidelines, which prioritize single-prefix forms to prevent misinterpretation.[13] Mathematically, a prefixed unit can be represented as the prefix factor multiplied by 10 raised to the power of the unit's exponent, but simplification to a single equivalent prefix is always required where possible. For a unit u with prefix p (where p = 10^k) raised to the power n, the expression is p × u^n = 10^{k n} × (base unit)^n, which should be restated using a single prefix corresponding to 10^{k n} applied to the base unit power. This approach aligns with the foundational rules for prefix application, ensuring expressions remain concise and unambiguous.[10]Typographical and Practical Considerations
Symbol Rendering and Special Cases
The micro- prefix, denoting a factor of 10^{-6}, is represented by the Greek letter μ (mu), which was adopted in the SI system to signify one millionth of a unit.[10] This symbol replaced earlier notations like mμ (milli-micro), which had been used historically for 10^{-9}, such as in the term millimicron for what is now the nanometer (nm); the change to single prefixes like nano- eliminated such double-prefix combinations for clarity and standardization.[10] An approved alternative rendering is the micro sign µ, particularly in digital contexts where the Greek μ may not be readily available, though the Greek form is preferred in formal SI documentation.[14] The choice of μ for micro- specifically addresses potential ambiguity with the symbol m, which denotes both the milli- prefix (10^{-3}) and the base unit meter; using a distinct Greek letter prevents confusion in expressions involving length or other units starting with m.[10] In textual contexts, SI rules recommend writing out the prefix name (e.g., "microgram") or using spacing and context to distinguish symbols, such as μm for micrometer versus mm for millimeter, ensuring unambiguous interpretation without additional modifiers.[14] Special cases in prefix usage include the absence of a dedicated symbol for 10^{0}, where the base unit stands alone without modification, as prefixes are intended only for decimal multiples or submultiples beyond unity.[10] In compound units involving multiplication or division, prefixes apply separately to each component unit; for example, micrograms per cubic meter is rendered as μg/m³, with μ attaching to g (gram) in the numerator and no prefix on m (meter) in the denominator unless scaling requires it, such as μg/(m³) to emphasize the structure.[14] International standards, such as those from the International Organization for Standardization (ISO 80000-1), specify that SI prefix symbols like μ must be rendered in upright (roman) typeface to distinguish them from italicized symbols for physical quantities (e.g., m for mass), promoting consistency across publications and avoiding typographical errors in scientific communication. This upright convention aligns with BIPM guidelines and applies globally, though some legacy or non-technical contexts may vary in adherence.[10]Input and Typesetting Methods
For entering the micro prefix symbol in digital documents, the preferred character is the Greek small letter mu at Unicode code point U+03BC, which is distinct from the legacy micro sign at U+00B5. On Windows systems, U+03BC can be input in applications like Microsoft Word by typing "03BC" followed by Alt+X, or via the Insert > Symbol dialog under the Greek and Coptic subset; alternatively, the legacy U+00B5 is entered using Alt+230 on the numeric keypad.[15] On macOS, Option+M produces the micro sign U+00B5 in most text editors; for the preferred Greek mu U+03BC, use the Character Viewer (Control+Command+Space, then search for "mu") or enable a Greek input source via System Settings > Keyboard > Input Sources.[16] In plain text environments without Unicode support, alternatives such as "u" (for micro) or the two-letter "mc" may be used temporarily, though these are not recommended for formal documents as they deviate from SI standards.[10] For web content, HTML entities include μ for U+03BC and µ for U+00B5.[17] In LaTeX typesetting, the siunitx package provides robust support for SI prefixes, enabling commands like \micro for the micro symbol and \kilo for kilo, typically within \si{...} for structured units such as \si{\micro\meter}.[18] Direct input of the symbol uses in math mode, but siunitx ensures upright roman rendering as required by SI conventions, avoiding italicization common in variables.[10] Prefixes like \mega or \nano are predefined, promoting consistency across documents.[19] Best practices emphasize upright (roman) fonts for all prefix symbols to align with SI guidelines, as italic fonts are reserved for variables.[10] For the micro symbol specifically, serif fonts are preferable over sans-serif to distinguish μ from the Latin "u," which can appear identical in some sans-serif typefaces like Arial, potentially causing readability issues.[20] In PDF generation or Microsoft Word, ensure UTF-8 encoding and embed fonts supporting Greek characters to prevent substitution errors during export or viewing.[21] Regarding accessibility, screen readers such as JAWS or NVDA typically announce U+03BC as "mu," providing phonetic clarity for the micro prefix in technical contexts, provided the document uses standard Unicode and avoids custom mappings.[22] For enhanced compatibility in complex documents, authors may include brief textual descriptions nearby or use ARIA attributes in web formats to specify "micro" explicitly if the default pronunciation is ambiguous.Applications to Units
In SI Base and Derived Units
Metric prefixes are systematically applied to the SI base units to express multiples and submultiples, enabling the representation of physical quantities across vast scales while maintaining the decimal nature of the system. For the base unit of mass, the kilogram (kg), prefixes such as mega- (M) yield the megagram (Mg), equivalent to kg, which is also the definition of the tonne (t = 1 Mg).[10] Submultiples like micro- (μ) produce the microgram (μg = g = kg), useful for small masses in chemistry and biology.[1] This scalability ensures that masses ranging from microscopic particles to large industrial quantities can be denoted concisely without altering the underlying unit coherence.[10] In length measurement, the metre (m) combines with prefixes to cover distances from the atomic to the astronomical. The kilometre (km) represents m, commonly used for road distances and geographical scales, such that 1 km = m.[1] At the smaller end, the nanometre (nm) denotes m, essential for describing wavelengths of light and molecular dimensions.[10] For time, the second (s) is prefixed as millisecond (ms = s) for brief events like reaction times, while the kilosecond (ks = s, approximately 16.7 minutes) is rarely used but available for longer intervals in scientific contexts.[1] Temperature in kelvin (K) does not typically employ prefixes for absolute values due to the scale's origin at absolute zero, but they are applied to temperature intervals or differences, such as millikelvin (mK = K) for cryogenic measurements.[10] Prefixes extend naturally to SI derived units, preserving the system's coherence by scaling the entire expression without introducing conversion factors. The joule (J), defined as J = kg·m²/s² for energy, becomes the kilojoule (kJ = J) for larger energies like those in nutrition or engineering, where 1 kJ = J.[1] For electric potential, the volt (V) uses kilo- to form the kilovolt (kV = V), standard in power transmission.[10] Frequency, measured in hertz (Hz), employs mega- for the megahertz (MHz = Hz), common in electronics and radio communications.[1] These applications demonstrate how prefixes facilitate precise expression across disciplines while upholding the SI's foundational structure.[10]In Non-SI and Historical Units
Metric prefixes are applied to certain non-SI units accepted for use with the International System of Units (SI), enhancing expressiveness for very large or small quantities without altering the core definitions of these units. The litre (L or l), equivalent to one cubic decimetre (10^{-3} m³), routinely incorporates prefixes such as milli- to form the millilitre (mL), which denotes one-thousandth of a litre and is widely used in medical, chemical, and laboratory contexts for precise volume measurements. Similarly, the electronvolt (eV), defined as the kinetic energy gained by an electron accelerated through a potential difference of one volt (approximately 1.602 \times 10^{-19} J), employs prefixes like kilo- (keV), mega- (MeV), and giga- (GeV) in particle physics and nuclear science to describe energy scales ranging from atomic interactions to high-energy collisions. These applications maintain coherence with SI practices while accommodating established non-SI nomenclature.[10] In the realm of energy measurement, the calorie (cal), historically defined as the energy required to raise the temperature of one gram of water by one degree Celsius (approximately 4.184 J), is accepted for use with the SI but does not officially permit metric prefixes, and its application is generally discouraged in favor of the joule to promote uniformity. Despite this, the kilocalorie (kcal), representing 1000 calories, persists in nutritional and dietary contexts as a practical scaling for metabolic energy, such as daily caloric intake, where it equates to the large calorie (Cal) used in food labeling. This informal prefix usage reflects historical conventions in biochemistry and public health, though international standards emphasize SI-derived units for scientific precision.[10][23] For angular measurements, the radian (rad), a dimensionless SI-derived unit, does not formally accept metric prefixes under SI rules, yet the milliradian (mrad or mr), equivalent to 0.001 radian, is commonly employed in practical applications like ballistics, optics, and surveying to quantify small angles with high resolution. In engineering and aerospace, such informal prefixing aids in specifying tolerances, such as angular deviations in guidance systems. Likewise, time units like the minute (min = 60 s) prohibit official prefixes, with no recognized form like "kmin," but the kilosecond (ks = 1000 s) is occasionally used in scientific contexts, such as astrophysics observations or long-duration experiments, to avoid cumbersome numerical values for intervals spanning roughly 16.7 minutes.[10][24][25] Historically, metric prefixes have been adapted informally to non-SI and imperial units in specialized fields, particularly engineering, where precision demands sub-unit scales. For instance, the microinch (μin), one-millionth of an inch, measures surface roughness in manufacturing and machining, with values like 16 μin indicating high-quality finishes achieved through grinding or honing, as specified in standards for components in aerospace and automotive industries. In nautical contexts, while traditional units like the nautical mile (approximately 1.852 km) do not directly incorporate prefixes, metric equivalents such as the kilometer facilitate scaling for distances in modern navigation charts and hydrographic surveys. These adaptations highlight the versatility of metric prefixes beyond strict SI boundaries, though they remain non-standard and context-specific.[26][2]Non-Standard Extensions
Obsolete and Deprecated Prefixes
The myria prefix, denoting a factor of , and its counterpart myrio for , were part of the original metric system established in 1795 but were officially deprecated by the 11th General Conference on Weights and Measures (CGPM) in 1960 to streamline the set of prefixes and eliminate redundancy with combinations such as hectokilo-.[2] These changes aligned the system with powers of 10 in increments of three orders of magnitude, favoring kilo- () and mega- () over myria- for larger multiples.[2] Another deprecated term is "micron," an informal name for the micrometer ( m) that was accepted until the 13th CGPM in 1967–1968, when it was abrogated to avoid confusion with the micro prefix (symbol μ) and to standardize unit nomenclature.[27][10] The International Bureau of Weights and Measures (BIPM) now recommends exclusively using the micrometre with symbol μm.[4] In some historical and regional contexts, particularly in long-scale numbering systems used in parts of Europe, the term "milliard" referred to , occasionally applied informally to metric units, but this usage is obsolete in the SI as it lacks a standardized prefix and can cause ambiguity with short-scale "billion."[4] The BIPM advises employing the official giga- prefix (G, ) instead to ensure global consistency.[4] These obsolete prefixes and terms persist in older scientific literature and engineering texts from before the 1960s revisions, but their use in new publications is discouraged by the BIPM to maintain coherence in the International System of Units.[4] Deprecation stemmed primarily from efforts to reduce ambiguity, eliminate non-decimal or redundant elements, and adapt to scientific needs for precise, universal scaling.[2]Recent and Proposed Additions
In November 2022, the 27th General Conference on Weights and Measures (CGPM) adopted four new metric prefixes to extend the range of the International System of Units (SI), marking the first such addition since 1991. These include ronna (symbol R, denoting 10^{27}) and quetta (symbol Q, denoting 10^{30}) for large-scale multiples, along with their corresponding submultiples ronto (symbol r, 10^{-27}) and quecto (symbol q, 10^{-30}).[7][2] The introduction of these prefixes addresses the growing demands of fields requiring expression of extremely large and small quantities. For instance, in exascale computing and data storage, the prefix ronna enables concise notation for vast datasets, such as 1 ronnabyte (RB) equivalent to 10^{27} bytes, which supports projections for AI systems and big data analytics anticipated in the 2030s. Similarly, quetta facilitates descriptions in large-scale physics and astronomy, where datasets from telescopes and simulations exceed the capacity of prior prefixes like zetta (10^{21}). At the submultiple end, ronto and quecto are tailored for quantum physics and particle measurements, accommodating scales below yocto (10^{-24}).[28][29] This expansion was driven by the insufficiency of existing prefixes for emerging technologies, with experts noting that without updates, scientific communication in genomics, cosmology, and high-performance computing would become cumbersome. The CGPM's decision ensures the SI remains adaptable to advancements projected over the next two to three decades.[3][8] These new prefixes were promptly incorporated into the 9th edition of the SI Brochure, with version 2.01 released in December 2022 by the International Bureau of Weights and Measures (BIPM), confirming their full integration into the official SI framework. The SI Brochure was last revised in August 2025, incorporating no further prefix changes.[10][30] Beyond official adoptions, informal discussions within metrology and scientific communities have explored further extensions for even larger scales. However, as of November 2025, no such proposals have achieved formal status or CGPM approval, remaining speculative and unofficial.Related Concepts and Distinctions
Similar Symbols and Abbreviations
In chemistry, the uppercase letter M commonly denotes molar concentration, defined as the amount of substance in moles per liter of solution, a usage established in standard chemical nomenclature. This symbol can potentially overlap with the metric prefix M for mega-, which indicates a factor of , as seen in units like megameter (Mm). However, such confusion is typically avoided because the mega prefix is always attached to a unit symbol or name, whereas standalone M in chemical contexts unambiguously refers to molarity.[31][2] The Greek letter mu (μ) serves as the symbol for the metric prefix micro-, denoting , and is widely used in scientific notation for quantities like micrometers (μm). Historically, μ also represented the non-SI unit "micron," an obsolete synonym for micrometer equivalent to one millionth of a meter. Although the term and standalone symbol for micron have been deprecated since the adoption of the International System of Units (SI) in 1960, legacy usage in fields like optics and filtration can still lead to ambiguity, prompting recommendations to use "micrometer" exclusively.[27] In electrical engineering and physics, the lowercase letter m functions both as the SI symbol for the base unit meter (length) and as the prefix milli-, indicating . This dual role requires careful contextual interpretation; for instance, m alone denotes meters, while m prefixed to another unit like ampere yields milliamperes (mA). No similar conflict arises with the symbol Ω, which exclusively represents the SI unit ohm (electrical resistance), distinct from the Greek letter omega (ω) used in angular frequency or other mathematical contexts, as metric prefixes do not employ Greek letters beyond μ.[32][2] In medical and pharmaceutical contexts, the abbreviation mcg is recommended for microgram ( grams) to mitigate handwriting ambiguities with the Greek mu (μ or µg), which can resemble mg (milligram) and lead to dosing errors by a factor of 1,000. This practice stems from safety guidelines aimed at preventing medication mishaps, particularly in prescriptions where illegible script exacerbates misinterpretation. Regional and domain-specific abbreviations can introduce further overlaps, such as Mb for megabit (a unit of digital information equivalent to bits, common in telecommunications) versus mb or Mb for millibar (a deprecated pressure unit equal to 100 pascals, still encountered in meteorology). The millibar, while non-SI, persists in weather reporting, but standardization efforts favor mbar or pascals to reduce confusion with data units.[33]Binary and Non-Decimal Prefixes
Binary prefixes, also known as IEC prefixes, are a set of unit prefixes designed specifically for multiples of powers of two, primarily to address ambiguities in computing and data storage where traditional metric prefixes (powers of ten) have been misapplied. Unlike standard metric prefixes, which strictly denote decimal multiples (10^n), binary prefixes represent 2^n, where n is a multiple of 10, such as 2^10 = 1024 for the kibi (Ki) prefix. This distinction arose because early computing conventions approximated 1024 as "kilo" (1000), leading to confusion in fields like RAM and storage capacities; for instance, a "kilobyte" could ambiguously mean either 1000 bytes (decimal) or 1024 bytes (binary).[34] The binary prefixes were standardized by the International Electrotechnical Commission (IEC) in Amendment 2 to IEC 60027-2, approved in December 1998 and published in 1999, with the second edition of the standard issued in November 2000. The third edition in 2005 added zebi and yobi, and the current standard, IEC 80000-13:2025 (published February 2025), supersedes prior versions and introduces robi and quebi to align with recent SI decimal prefixes.[34][35] These prefixes originated in the 1990s amid growing concerns over inconsistent usage in information technology, where binary scales are fundamental due to the base-2 nature of digital systems. Although not incorporated into the International System of Units (SI), which reserves prefixes for decimal powers, the IEC recommends binary prefixes for unambiguous communication in binary contexts, such as data processing and transmission. The names are formed by adding "bi" (for binary) to the first two letters of corresponding SI prefix names, with symbols appending "i" (e.g., kibi from kilo).[35] The following table lists the standard binary prefixes, their factors, names, and symbols as defined in IEC 80000-13:2025:| Factor | Name | Symbol |
|---|---|---|
| 2¹⁰ | kibi | Ki |
| 2²⁰ | mebi | Mi |
| 2³⁰ | gibi | Gi |
| 2⁴⁰ | tebi | Ti |
| 2⁵⁰ | pebi | Pi |
| 2⁶⁰ | exbi | Ei |
| 2⁷⁰ | zebi | Zi |
| 2⁸⁰ | yobi | Yi |
| 2⁹⁰ | robi | Ri |
| 2¹⁰⁰ | quebi | Qi |