The gallon currently has two definitions, in the imperial system and in the US customary system.

Historically, there were many definitions and redefinitions: see § Sizes of gallons for details.

The British imperial gallon (frequently called simply "gallon") is defined as exactly 4.54609 litres.^[3] It is used in Britain and some other Commonwealth countries, and until 1976 was defined as the volume of water at 62 °F (16.7 °C)^[4][5] whose mass is 10 pounds (4.5359237 kg).

There are four imperial quarts in a gallon, two imperial pints in a quart, and 20 imperial fluid ounces in an imperial pint,^[3] making an imperial fluid ounce ⁠1/160⁠ of an imperial gallon.

The US gallon (frequently called simply "gallon") is legally defined as exactly 231 cubic inches, i.e. 3.785411784 litres.^[6][7]

A US gallon contains 3.785411784 kg (8.3454 lb) of water at 3.983 °C (39.169 °F), making it 83.26742% of an imperial gallon. There are four quarts in a gallon, two pints in a quart and 16 US fluid ounces in a US pint, making the US fluid ounce ⁠1/128⁠ of a US gallon.

In order to overcome the effects of expansion and contraction with temperature when using a gallon to specify a quantity of material for purposes of trade, it is common to define the temperature at which the material will occupy the specified volume. For example, the volume of petroleum products^[8] and alcoholic beverages^[9] are both referenced to 60 °F (15.6 °C) in government regulations.

As of 2021, the imperial gallon continues to be used as the standard petrol unit on 10 Caribbean island groups, consisting of:

• four British Overseas Territories (Anguilla,^[10] the British Virgin Islands,^[11][12] the Cayman Islands,^[13] and Montserrat)^[14][15] and
• six countries (Antigua and Barbuda,^[16] Dominica,^[17] Grenada,^[18][19] Saint Christopher and Nevis,^[20] Saint Lucia,^[21] and Saint Vincent and the Grenadines^[22]).

All 12 of the Caribbean islands use miles per hour for speed limits signage, and drive on the left side of the road.

The Dubai Electricity and Water Authority switched Dubai's water billing from imperial gallons to cubic metres in March 2025.^[23]

The United Arab Emirates ceased selling petrol by the imperial gallon in 2010 and switched to the litre, with Guyana following suit in 2013.^[24][25][26] In 2014, Myanmar switched from the imperial gallon to the litre.^[27]

Antigua and Barbuda has proposed switching to selling petrol by litres since 2015.^[16][28]

In the European Union the gallon was removed from the list of legally defined primary units of measure catalogue in the EU directive 80/181/EEC for trading and official purposes, effective from 31 December 1994. Under the directive the gallon could still be used, but only as a supplementary or secondary unit.^[29]

As a result of the EU directive Ireland and the United Kingdom passed legislation to replace the gallon with the litre as a primary unit of measure in trade and in the conduct of public business, effective from 31 December 1993 and 30 September 1995, respectively.^{[30][31][32][33]} Though the gallon has ceased to be a primary unit of trade, it can still be legally used in both the UK and Ireland as a supplementary unit. However, barrels and large containers of beer, oil and other fluids are commonly measured in multiples of an imperial gallon.

Miles per imperial gallon is used as the primary fuel economy unit in the United Kingdom and as a supplementary unit in Canada on official documentation.^[34][35][36]

Other than the United States, petrol is sold by the US gallon in eight other countries and four US territories:

• the Caribbean country of Haiti,
• the Central American countries of Belize, El Salvador and Guatemala,
• the Pacific Ocean countries of Marshall Islands, Federated States of Micronesia, and Palau, which are associated countries of the United States,
• the African country of Liberia, a former protectorate of the United States,^[37] and
• the US territories of American Samoa,^[38] the Northern Mariana Islands,^[39] Guam,^[40] and the US Virgin Islands.^[41] Puerto Rico ceased selling petrol by the US gallon in 1980.^[42]

Both the imperial gallon and the US gallon are used in the Turks and Caicos Islands, due to an increase in tax duties which was disguised by levying the same duty on the US gallon (3.79 L) as was previously levied on the imperial gallon (4.55 L),^[43] and the Bahamas.^[44][45]

In some parts of the Middle East, such as the United Arab Emirates and Bahrain, 18.9-litre water cooler bottles are marketed as five-gallon bottles.^[46]

Both the US gallon and imperial gallon are divided into four quarts (quarter gallons), which in turn are divided into two pints, which in turn are divided into two cups (not in customary use outside the US), which in turn are further divided into two gills. Thus, both gallons are equal to four quarts, eight pints, sixteen cups, or thirty-two gills.

There is a difference in that the imperial gill is further divided into five fluid ounces, whereas the US gill is divided into four fluid ounces: this means that an imperial fluid ounce is ⁠1/20⁠ of an imperial pint or ⁠1/160⁠ of an imperial gallon, while a US fluid ounce is ⁠1/16⁠ of a US pint or ⁠1/128⁠ of a US gallon.

As an imperial fluid ounce is 96.076% of a US fluid ounce, this means that one imperial gallon, quart, pint, cup and gill are all equal to 1.20095 of their US counterparts.

Historically, a common bottle size for liquor in the US was the "fifth", i.e. one-fifth of a US gallon (or 0.08% more than a "reputed quart", one-sixth of an imperial gallon). While spirit sales in the US were switched to metric measures in 1976, a 750 mL bottle is still sometimes known as a "fifth".^[47][48]

The US dry gallon was defined as one-eighth of a US Winchester bushel of exactly 2,150.42 cubic inches, i.e. 268.8025 cubic inches or 4.40488377086 L.^[49]

The US dry gallon is no longer used, and is no longer included in the relevant statute, which goes from the dry quart to the peck.^[50]

The term derives most immediately from galun, galon in Old Norman French,^[51] but the usage was common in several languages, for example jale in Old French and gęllet (bowl) in Old English. This suggests a common origin in Romance Latin, but the ultimate source of the word is unknown.^[52]

The gallon originated as the basis of systems for measuring wine and beer in England. The sizes of gallon used in these two systems were different from each other: the first was based on the wine gallon (equal in size to the US gallon), and the second one the ale gallon (1.65% larger than the imperial gallon).

By the end of the 18th century, there were three definitions of the gallon in common use:

• The corn gallon (or Winchester gallon) of about 268.8025 cubic inches (≈4.404884 L),
• The wine gallon (or Queen Anne's gallon) of 231 cubic inches^[53] (3.785411784 L), and
• The ale gallon of 282 cubic inches (4.621152048 L).

The corn or dry gallon was used in the United States for grain and other dry commodities. It was one-eighth of the (Winchester) bushel, originally defined as a cylindrical measure of ⁠18+1/2⁠ inches in diameter and 8 inches in depth, which made the bushel 8 in × (⁠9+1/4⁠ in)² × π ≈ 2,150.42017 cubic inches. The bushel was later redefined to be 2,150.42 cubic inches exactly, thus making its gallon exactly 268.8025 in³ (4.40488377086 L); in previous centuries, there had been a corn gallon of between 271 and 272 cubic inches.

The wine gallon was legally adopted as the standard US gallon in 1836. Some sources relate this to the volume occupied by eight medieval merchants' pounds of wine: this was at one time defined as the volume of a cylinder 6 inches deep and 7 inches in diameter, i.e. 6 in × (⁠3+1/2⁠ in)² × π ≈ 230.90706 cubic inches. It was redefined in 1706 during the reign of Queen Anne as being exactly 231 in³, the earlier definition with π being approximated as ⁠22/7⁠.

$${\displaystyle \pi r^{2}h\approx {\frac {22}{7}}\times \left({\frac {7~\mathrm {in} }{2}}\right)^{2}\times 6~\mathrm {in} =231~\mathrm {in} ^{3}.}$$

Although the wine gallon had been used for centuries for import duty purposes, there was no legal standard of it in the Exchequer, and a smaller gallon 224 in³ (3.670702336 L) was actually in use, which required this statute to resolve these issues: 231 in³ remains the definition of a gallon in the US today.

In 1824, Britain adopted the imperial gallon, and abolished all other gallons in favour of it. The imperial gallon was defined as the volume of 10 pounds of distilled water weighed in air with brass weights with the barometer standing at 30 inches of mercury (100 kilopascals; 15 pounds per square inch) and at a temperature of 62 °F (16.7 °C), which was calculated as 277.274 in³ (or 4.543706784 L to ten significant figures).

This value lasted until 1889, when an Order in Council of November 28 of that year redefined the imperial gallon as 277.463 in³ (or 4.546803939 L to ten significant figures).

In 1963, the definition was again refined as the space occupied by 10 pounds of distilled water of density 0.998859 g/mL weighed in air of density 0.001217 g/mL against weights of density 8.136 g/mL (the original "brass" was refined as the densities of brass alloys vary depending on metallurgical composition), which was calculated as 4.546091879 L (≈ 277.4195 in³) to ten significant figures.^[4]

The definition of exactly 4.54609 cubic decimetres (also 4.54609 L or ≈ 277.4194 in³) came after the litre was redefined in 1964. This was adopted shortly afterwards in Canada, and adopted in 1976 in the United Kingdom.^[4]

Historically, gallons of various sizes were used in many parts of Western Europe. In these localities, it has been replaced as the unit of capacity by the litre.