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A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric units, used in every country globally. In the United States the U.S. customary units are also in use. British Imperial units are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided into SI and non-SI units.[1][2][3]
Before the establishment of the decimal metric system in France during the French Revolution in the late 18th century,[4] many units of length were based on parts of the human body.[5][6]
The Nippur cubit was one of the oldest known units of length. The oldest known metal standard for length corresponds to this Sumerian unit and dates from 2650 BCE.[7][8] This copper bar was discovered in Nippur, on the banks of the Euphrates, and is kept in the Istanbul Archaeological Museum. Archaeologists consider that this 51.85 centimetres long unit was the origin of the Roman foot. Indeed, the Egyptians divided the Sumerian cubit into 28 fingers and 16 of these fingers gave a Roman foot of 29.633 cm.[8][6]The base unit in the International System of Units (SI) is the meter, defined as "the length of the path travelled by light in vacuum during a time interval of 1⁄299792458 seconds."[9] It is approximately equal to 1.0936 yd. Other SI units are derived from the meter by adding prefixes, as in millimeter or kilometer, thus producing systematic decimal multiples and submultiples of the base unit that span many orders of magnitude. For example, a kilometer is 1000 m.
In the centimeter–gram–second system of units, the basic unit of length is the centimeter, or 1⁄100 of a meter. Other non-SI units are derived from decimal multiples of the meter.
| Name | Symbol | SI value |
|---|---|---|
| fermi | fm | 1 femtometer |
| ångström | Å | 100 picometers |
| micron | μm | 1 micrometer |
| Norwegian/Swedish mil or myriameter | 10,000 meters | |
| x unit | xu | 0.1 picometer |
The basic unit of length in the imperial and U.S. customary systems is the yard, defined as exactly 0.9144 m by international treaty in 1959.[2][10]
Common imperial units and U.S. customary units of length include:[11]
In addition, the following are used by sailors:
Aviators use feet for altitude worldwide (except in Russia and China) and nautical miles for distance.[citation needed]
Surveyors in the United States continue to use:
The Australian building trades adopted the metric system in 1966 and the units used for measurement of length are meters (m) and millimeters (mm). Centimeters (cm) are avoided as they cause confusion when reading plans. For example, the length two and a half meters is usually recorded as 2500 mm or 2.5 m; it would be considered non-standard to record this length as 250 cm.[12][13]
American surveyors use a decimal-based system of measurement devised by Edmund Gunter in 1620. The base unit is Gunter's chain of 66 feet (20 m) which is subdivided into 4 rods, each of 16.5 ft or 100 links of 0.66 feet. A link is abbreviated "lk", and links "lks", in old deeds and land surveys done for the government.
Astronomical measure uses:
In atomic physics, sub-atomic physics, and cosmology, the preferred unit of length is often related to a chosen fundamental physical constant, or combination thereof. This is often a characteristic radius or wavelength of a particle. Some common natural units of length are included in this table:
| Atomic property | Symbol | Length, in meters | Reference |
|---|---|---|---|
| The classical electron radius | re | 2.817940285(31)×10−15 | [18] |
| The Compton wavelength of the electron | λC | 2.426310215(18)×10−12 | [18] |
| The reduced Compton wavelength of the electron | 3.8615926764(18)×10−13 | [19] | |
| The Compton wavelength (or reduced Compton wavelength) of any fundamental particle | |||
| The Bohr radius of the hydrogen atom (Atomic unit of length) | a0 | 5.291772083(19)×10−11 | [18] |
| The reduced wavelength of hydrogen radiation | 1 / R∞ | 9.112670505509(83)×10−8 | [18] |
| The Planck length | 𝓁P | 1.616199(97)×10−35 | [20] |
| Stoney unit of length | lS | 1.381×10−35 | |
| Quantum chromodynamics (QCD) unit of length | lQCD | 2.103×10−16 | |
| Natural units based on the electronvolt | 1 eV−1 | 1.97×10−7 |
Archaic units of distance include:
In everyday conversation, and in informal literature, it is common to see lengths measured in units of objects of which everyone knows the approximate width. Common examples are:
Horse racing and other equestrian activities keep alive:
