Gallon

Worldwide usage

Imperial gallon

The imperial gallon continues to be used as the standard petrol unit in four British Overseas Territories (Anguilla, the British Virgin Islands, the Cayman Islands, and Montserrat), and four countries (Antigua and Barbuda, Grenada, Saint Lucia, and Saint Vincent and the Grenadines).

The United Arab Emirates ceased selling petrol by the imperial gallon in 2010 and switched to the litre, with Guyana following suit in 2013. The Dubai Electricity and Water Authority switched Dubai's water billing from imperial gallons to cubic metres in March 2025.

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.

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.

US gallon

Gasoline is sold by the US gallon in four US territories (American Samoa, the Northern Mariana Islands, Guam, and the US Virgin Islands) and in ten countries:

• six countries in the Americas: Colombia, Ecuador, Guatemala, Haiti, Peru, and the United States;
• three associated states of the United States Pacific Ocean countries: Marshall Islands, the Federated States of Micronesia, and Palau; and
• the African country Liberia, a former protectorate of the United States.

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 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, or thirty-two gills.

There is a difference in that the imperial gill is 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".

US dry gallon

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.

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.

History

The term derives most immediately from galun, galon in Old Norman French, 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. Gallonage is the term used for a capacity or amount measured in gallons.

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 was based on 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 (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 until the 1990s. 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 ≈ 2150.42017 cubic inches. The bushel was later redefined to be 2150.42 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.

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

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 of 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 (≈ 277.41955 in³) to ten significant figures.

The definition of exactly 4.54609 cubic decimetres (also 4.54609 L or ≈ 277.41943 in³) came after the litre was redefined in 1964: this value was adopted shortly afterwards in Canada, and was adopted in 1976 in the United Kingdom.

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.