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The gallon is a unit of volume in British and United States customary units.
The imperial gallon (imp gal) is defined as , and is or was used in the United Kingdom and its former colonies, including Ireland, Canada, Australia, New Zealand, India, South Africa, Malaysia and some Caribbean countries, while the US gallon (US gal) is defined as , and is used in the United States and some and Caribbean countries.
There are four gills in a pint, two in a quart, and four ( quarter gallons) in a gallon, with the imperial gill being divided into five imperial fluid ounces and the US gill being divided into four US fluid ounces: this, and a slight difference in the sizes of the imperial fluid ounce and the US fluid ounce, give different sizes for the imperial gallon and US gallon.
The IEEE standard symbol for both the imperial and US gallons is gal,IEEE Std 260.1–2014 not to be confused with the gal (symbol: Gal), a CGS unit of acceleration.
Historically, there were many definitions and redefinitions: see for details.
There are four Imperial units in a gallon, two in a quart, and 20 imperial in an imperial pint, making an imperial fluid ounce of an imperial gallon.
A US gallon contains of water at , making it % of an imperial gallon. There are four in a gallon, two in a quart and 16 US fluid ounces in a US pint, making the US fluid ounce 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 and alcoholic beverages are both referenced to in government regulations.
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.
The United Arab Emirates ceased selling petrol by the imperial gallon in 2010 and switched to the litre, with Guyana following suit in 2013. In 2014, Myanmar switched from the imperial gallon to the litre.
Antigua and Barbuda has proposed switching to selling petrol by litres since 2015.
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.
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. 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.
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 of an imperial pint or of an imperial gallon, while a US fluid ounce is of a US pint or of a US gallon.
As an imperial fluid ounce is % of a US fluid ounce, this means that one imperial gallon, quart, pint, cup and gill are all equal to 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".E. Frank Henriques, The Signet Encyclopedia of Wine, p. 298
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. Authorized tables , US Code, Title 15, ch. 6, subchapter I, sec. 205, accessed 19 July 2008.
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 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 inches in diameter and 8 inches in depth, which made the bushel . The bushel was later redefined to be 2,150.42 cubic inches exactly, thus making its gallon exactly (); 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 merchant 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. . It was redefined in 1706 during the reign of Queen Anne as being exactly , the earlier definition with being approximated as .
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 () was actually in use, which required this statute to resolve these issues: 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 and at a temperature of , which was calculated as (or 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 (or to ten significant figures).
In 1963, the definition was again refined as the space occupied by 10 pounds of distilled water of density weighed in air of density against weights of density (the original "brass" was refined as the densities of brass alloys vary depending on metallurgical composition), which was calculated as (≈ ) to ten significant figures.
The definition of exactly cubic decimetres (also or ≈ ) came after the litre was redefined in 1964. This was adopted shortly afterwards in Canada, and adopted in 1976 in the United Kingdom.
US gallon
The US gallon (frequently called simply "gallon") is legally defined as exactly 231 , i.e. .
Worldwide usage
units used in the world:
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Imperial gallon
As of 2021, the imperial gallon continues to be used as the standard petrol unit on 10 Caribbean island groups, consisting of:
US gallon
Other than the United States, petrol is sold by the US gallon in seven other countries and four US territories:
Both imperial and US gallon
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), and the Bahamas.
Legacy
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.
Relationship to other units
Both the US gallon and imperial gallon are divided into four quarts ( quarter gallons), which in turn are divided into two , 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.
US dry gallon
The US dry gallon was defined as one-eighth of a US Winchester bushel of exactly cubic inches, i.e. 268.8025 cubic inches or .
History
The term derives most immediately from galun, galon in Old Norman, 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 Vulgar Latin, but the ultimate source of the word is unknown.
Sizes of gallons
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.
+ Comparison of gallons Statute of 5 Queen Anne (1706)
UK wine gallon
US gallon (legally adopted 1836 US)8.3454 0.04 Imperial gallon (adopted 1964 Canada, adopted 1976 UK) 10.0224 Roman congius 7.8035 0.01 Irish gallon (1495, re-confirmed 1736) 7.8396 0.24 Preserved at the Guildhall, London (old UK wine gallon) 8.0925 0.6 Jersey gallon (1562) 8.7195 0.32 Guernsey gallon (17th-century origins until 1917) 9.4048 0.06 Ancient Rumford quart (1228) 9.5665 0.1 Exchequer (Henry VII, 1497, with rim) 9.5918 0.01 Ancient Rumford (1228) 9.6189 0.39 Henry VII (Winchester) corn gallon (1497) 9.6966 0.08 Winchester, statute of 13 & 14 William III
Corn gallon
US dry gallon (no longer used, no longer listed in the relevant statute)9.7111 Elizabeth I corn gallon (1601) 9.7544 0.0072 Exchequer (1601, E.) (old corn gallon) 9.7905 0.23 William III corn gallon (1688) 9.8266 0.05 Statute of 12 Anne (coal gallon), also equal to corn gallons 10.0146 0.08 Imperial gallon, as originally determined in 1824 10.0172 0.05 Imperial gallon as re-determined in 1895 and defined in 1963 10.0224 Imperial gallon as defined by Order in Council of 28 November 1889 10.024 0.015 Exchequer (Henry VII, with copper rim) 10.0434 0.12 Exchequer (1601 and 1602 pints) 10.0578 0.22 Exchequer (1601 quart) 10.1156 0.07 Treasury (beer and ale gallon pre-1824) 10.1879 0.02
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