The gallon (abbreviation "gal"),^[1] with the US and imperial gallon, is a measure of volume. Historically it has had many definitions, but there are three definitions in current use: the imperial gallon (≈ 4.546 L) which since metrication is used colloquially in the United Kingdom and semi-officially within Canada, the United States (liquid) gallon (≈ 3.79 L), and the lesser used US dry gallon (≈ 4.40 L). The gallon, be it the imperial or US gallon, is sometimes found in other English-speaking countries.

Definitions[link]

A one-US-gallon gas can showing "U.S. gallon" marking (for US use), imperial gallons (for Canadian use) and litres.

There is one gallon defined in the imperial system but two (liquid and dry) in the US customary system.

The imperial gallon

The imperial (UK) gallon, defined as 4.54609 L, is used in some Commonwealth countries and was originally based on the volume of 10 pounds of water at 62 °F (17 °C). (A US liquid gallon of water weighs about 8.33 pounds at the same temperature.) The imperial fluid ounce is defined as ¹⁄₁₆₀ of an imperial gallon.

The US liquid gallon

This gallon is defined as 231 cubic inches,^[2] and is equal to exactly 3.785411784 litres or about 0.13368 cubic feet. This is the most common definition of a gallon in the United States. The US fluid ounce is defined as ¹⁄₁₂₈ of a US liquid gallon.

The US dry gallon

This gallon is one-eighth of a US Winchester bushel of 2150.42 cubic inches, thus it is equal to exactly 268.8025 cubic inches or 4.40488377086 L. The US dry gallon is less commonly used, and is not listed in the relevant statute, which jumps from the dry quart to the peck.^[2]

Worldwide usage of gallons[link]

The imperial gallon is used in everyday life (and in advertising) in Canada, the United Kingdom and Ireland, including alongside litres per 100 km in advertisements and other official publications for expressing fuel economy figures in miles per gallon.^[3][4][5]

The gallon was removed from the list of legally defined primary units of measure catalogued in the EU directive 80/181/EEC, for trading and official purposes, with effect from 31 December 1994. Under the directive the gallon could still be used – but only as a supplementary or secondary unit.^[6] One of the impacts of this directive was that the United Kingdom amended its own 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 30 September 1995.^[7][8][9]

Ireland also passed legislation in response to the EU directive with the effective date being 31 December 1993.^[10] Though the gallon has ceased to be the legally defined primary unit, it can still be legally used in both the UK and Ireland as a supplementary unit.

The Imperial gallon is used as a unit of measure for fuel in Guyana.^[11]

The US gallon is used as a unit of measure for fuel in Liberia, Belize, Colombia, The Dominican Republic, Ecuador, El Salvador, Grenada, Guatemala, Haiti, Honduras, Myanmar (Burma), Nicaragua, Panama, Peru, and the United States.^[11]

Relationship to other units[link]

The gallons in current use are subdivided into eight pints or four quarts (quarter gallon). Pints are further subdivided into cups (US) (half of a pint) and fluid ounces, while liquid gallons are also subdivided into 32 gills, i.e. a quarter of a pint. The sub-units of pint and fluid ounce (and especially cup), despite having the same name in both imperial and US units, differ in volume and are therefore not interchangeable. The principal difference is that the imperial pint contains 20 imperial fluid ounces, whereas the US pint contains 16 US fluid ounces. A US fluid ounce is approximately 4% bigger than an Imperial fluid ounce, and therefore they are often used interchangeably, whereas US and imperial pints and gallons are sufficiently different that they should not be used interchangeably.

History[link]

The term derives most immediately from galun, galon in Old Northern French, but the usage was common in several languages, for example jale in Old French and gęllet (bowl) in Old English. This implies a common origin in Romance Latin, but the ultimate source of the word is unknown.^[12] The gallon originated as the base of systems for measuring wine, and ale 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 on either the ale gallon or the smaller imperial gallon.

By the end of the 18th century, three definitions of the gallon were in common use:^{[citation needed]}

• The corn gallon, or Winchester gallon, of about 268.8 cubic inches (≈ 4.405 L),
• The wine gallon, or Queen Anne's gallon, which was 231 cubic inches^[13] (≈ 3.79 L), and
• The ale gallon of 282 cubic inches (≈ 4.62 L).

The corn or dry gallon was used in the United States until recently for grain and other dry commodities. It is one-eighth of the (Winchester) bushel, originally a cylindrical measure of 18 ¹⁄₂ inches in diameter and 8 inches in depth. That made the dry gallon (9 ¹⁄₄)² × π cubic inches ≈ 268.80252 cu in. The bushel, which like dry quart and pint still sees some use, was later defined to be 2150.42 cubic inches exactly, making its gallon exactly 268.8025 cu in (4.40488377086 L). In previous centuries, there had been a corn gallon of around 271 to 272 cubic inches.

The wine, fluid, or liquid gallon has been the standard US gallon since the early 19th century. The wine gallon, which some sources relate to the volume occupied by eight medieval merchant pounds of wine, was at one time defined as the volume of a cylinder six inches deep and seven inches in diameter, i.e. 6 in × (3 ¹⁄₂ in)² × π ≈ 230.907 06 cubic inches. It had been redefined during the reign of Queen Anne, in 1706, as 231 cubic inches exactly (3 × 7 × 11 in), which is the result of the earlier definition with π approximated to ²²⁄₇. 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 cu in) was actually in use, so this statute became necessary. It remains the US definition today.

In 1824, Britain adopted a close approximation to the ale gallon known as the imperial gallon and abolished all other gallons in favour of it. Inspired by the kilogram-litre relationship, the imperial gallon was based on the volume of 10 pounds of distilled water weighed in air with brass weights with the barometer standing at 30 inches of mercury and at a temperature of 62 °F. In 1963, this definition was 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. This works out at approximately 4.5460903 L (277.41945 cu in). The metric definition of exactly 4.54609 cubic decimetres (also 4.54609 L after the litre was redefined in 1964, ≈ 277.419433 cu in) was adopted shortly afterwards in Canada, but from 1976 the conventional value of 4.546092 L was used in the United Kingdom until the Canadian convention was adopted in 1985.

Comparison of historic gallons[link]

See also[link]

• Comparison of the imperial and US customary measurement systems
• English units of wine casks
• English units of brewery casks

References[link]