The gallon (abbreviation "gal"), is a measure of volume that was used in many parts of Western Europe and is still used in the United States. Historically its value differed depending on locality and commodity. In most localities it has been replaced by the litre, but three variants in current use: the imperial gallon (≈ 4.546 L) which since metrication is used colloquially in the United Kingdom and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.
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 litres, 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 8.34 pounds at the same temperature.) The imperial fluid ounce is defined as 1⁄160 of an imperial gallon.
- The US liquid gallon
The US gallon, which is equal to 3.785411784 litres is legally defined as 231 cubic inches. There are four quarts in a gallon, two pints in a quart and sixteen fluid ounces in a pint. 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, for purposes of trade, the volume of petroleum products and alcoholic beverages are both referenced to 60 °F (16 °C).
- 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.
Worldwide usage of gallons 
The imperial gallon is used in everyday life (and in advertising) in the United Kingdom and Ireland, and less frequently in Canada, including fuel economy expression in advertisements and other official publications. Gallons used in fuel economy expression in Canada is imperial gallons.
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. 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.
Ireland also passed legislation in response to the EU directive with the effective date being 31 December 1993. 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.
A German Government report on fuel prices dated 2010/11 stated that the Imperial gallon is used as a unit of measure for fuel in Guyana, Antigua and Barbuda and formerly in the United Arab Emirates. The United Arab Emirates switched to using litres on 1 January 2012. The US gallon is used in Liberia, Belize, Colombia, The Dominican Republic, Ecuador, El Salvador, Grenada, Guatemala, Haiti, Honduras, Myanmar (Burma), Nicaragua, Panama, Peru, Puerto Rico and the United States .
Ten-gallon hat 
Some cowboy hats have been called "ten-gallon" hats. The term came into use about 1925. The Stetson hat company boasted that the tight weave of most Stetsons hats made them sufficiently waterproof to be used as a bucket. Early print advertising by Stetson showed a cowboy giving his horse a drink of water from a hat. However, even the Stetson company notes that a "ten-gallon" hat holds only 3 quarts (about 3 L instead of 40 L).
Relationship to other units 
Both the US liquid and imperial gallon are divided into four quarts (quarter gallons), which in turn are divided into two pints. These pints are divided into two cups (though the imperial cup is rarely used now), which in turn are divided into two gills (gills are also rarely used). Thus a gallon is equal to four quarts, eight pints, sixteen cups or thirty-two gills. The imperial gill is further divided into five fluid ounces whereas the US gill is divided into four fluid ounces. Thus an imperial fluid ounce is 1⁄20 of an imperial pint or 1⁄160 of an imperial gallon whilst a US fluid ounce is 1⁄16 of a US pint or 1⁄128 of a US gallon.
The imperial gallon, quart, pint, cup and gill are approximately 20% larger than their US counterparts and are therefore not interchangeable. The imperial fluid ounce, on the other hand, is only 4% smaller than the US fluid ounce and therefore they are often used interchangeably.
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. The gallon originated as the base 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 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:
- 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 (≈ 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 in × 7 in × 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|
(gallons per cubic foot)
@ 62 °F)
|(cu in)||(L or dm3)||Diameter
|216||≈ 3.5396||Roman congius||8||7.8||5||11||0.01|
|224||≈ 3.6707||preserved at the Guildhall, London (old UK wine gallon)||7.71||8.09||9||3.5||0.6|
|231||3.785411784||statute of 5th of Queen Anne (US wine gallon, standard US gallon)||7.48||8.33||7||6||0.04|
|264.8||≈ 4.3393||ancient Rumford quart (1228)||6.53||9.57||7.5||6||0.1|
|265.5||≈ 4.3508||Exchequer (Henry VII, 1497, with rim)||6.51||9.59||13||2||0.01|
|266.25||≈ 4.3631||ancient Rumford (1228)|
|268.8025||4.40488377086||Winchester, statute 13 + 14 by William III (corn gallon, old US dry gallon)||6.43||9.71||18.5||1||0.00001|
|271||≈ 4.4409||Exchequer (1601, E.) (old corn gallon)||6.38||9.79||4.5||17||0.23|
|272||≈ 4.4573||corn gallon (1688)|
|277.18||≈ 4.5422||statute 12 of Anne (coal gallon) = 33/32 corn gallons||6.23||10|
|277.274||4.543460||Imperial Gallon (1824) as originally evaluated.||6.23||10|
|277.419433 (ca.)||4.54609||standard imperial gallon (metric) (1964 Canada gallon, 1985 UK gallon)||6.23||10||5⅔||11||0.0002%|
|≈ 277.419555||4.546092||Imperial gallon (1895) Re-determined in 1895, as defined in 1963.||6.23||10|
|278||≈ 4.5556||Exchequer (Henry VII, with copper rim)||6.21||10.04|
|278.4||≈ 4.5622||Exchequer (1601 and 1602 pints)||6.21||10.06|
|280||≈ 4.5884||Exchequer (1601 quart)||6.17||10.1|
|282||≈ 4.6212||Treasury (beer and ale gallon)||6.13||10.2|
|Look up gallon in Wiktionary, the free dictionary.|
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