The gallon is a unit of measurement for volume and fluid capacity in both the US customary units and the British imperial systems of measurement. Three significantly different sizes are in current use: the imperial gallon defined as 4.54609 litres, which is used in the United Kingdom, Canada, and some Caribbean nations; the US gallon defined as 231 cubic inches (exactly 3.785411784 litres), which is used in the US and some Latin American and Caribbean countries; and the least-used US dry gallon defined as 1/ US bushel (4.405 L). There are four quarts in a gallon and eight pints in a gallon; they too have different sizes in different systems.
The IEEE standard symbol for the gallon is gal.
The gallon currently has one definition in the imperial system, and two definitions (liquid and dry) in the US customary system. Historically, there were many definitions and redefinitions.
English system gallonsEdit
There were more than a few systems of liquid measurements in the pre-1884 United Kingdom.
- Winchester or Corn Gallon was 272 in3 (157 fl oz) (1697 Act 8 & 9 Will III c22)
- Henry VII (Winchester) corn gallon from 1497 onwards was 154.80 fl oz
- Elizabeth I corn gallon from 1601 onwards was 155.70 fl oz
- William III corn gallon from 1697 onwards was 156.90 fl oz
- Old English (Elizabethan) Ale Gallon was 282 in3 (162 fl oz) (1700 Act 11 Will III c15)
- Old English (Queen Anne) Wine Gallon was standardized as 231 in3 (133 fl oz) in the 1706 Act 5 Anne c27, but it differed before that:
- London 'Guildhall' gallon (before 1688) was 129.19 fl oz
- Jersey gallon (from 1562 onwards) was 139.20 fl oz
- Guernsey gallon (17th century origins till 1917) was 150.14 fl oz
- Irish Gallon was 217 in3 (125 fl oz) (1495 Irish Act 10 Hen VII c22 confirmed by 1736 Act Geo II c9)
The imperial (UK) gallon is now defined as exactly 4.54609 litres (about 277.42 cubic inches). It is used in some Commonwealth countries. Until 1976 it was based on the volume of 10 pounds (approximately 4.54 kg) of water at 62 °F (17 °C). There are four quarts in a gallon, the imperial pint is defined as 0.56826125 litres (i.e. 1/ gallon) and there are 20 imperial fluid ounces in an imperial pint.
US liquid gallonEdit
The US gallon is legally defined as 231 cubic inches, which is exactly 3.785411784 litres. A US liquid gallon of water weighs about 8.34 pounds or 3.78 kilograms at 62 °F (17 °C), making it about 16.6% lighter than the imperial gallon. There are four quarts in a gallon, two pints in a quart and 16 US fluid ounces in a US pint, which makes the US fluid ounce equal to 1/ 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 60 °F (15.6 °C) in government regulations.
US dry gallonEdit
This dry measure is one-eighth of a US Winchester bushel of 2150.42 cubic inches; it is therefore equal to exactly 268.8025 cubic inches or about 4.405 L. The US dry gallon is not used in commerce, and is not listed in the relevant statute, which jumps from the dry quart to the peck.
Worldwide usage of gallonsEdit
Despite its status as a U.S. territory, and unlike American Samoa, the Northern Mariana Islands, Guam, and the U.S. Virgin Islands, Puerto Rico ceased selling gasoline by the US gallon in 1980.
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 effects 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.
The United Arab Emirates started selling gasoline by the litre in 2010, along with Guyana, and Panama in 2013. The two former had used the Imperial gallon and the latter the US gallon until they switched.Myanmar (Burma) switched from Imperial gallon to litre sales before 2014.
The Imperial gallon continues to be used as a unit of measure in Anguilla, Antigua and Barbuda, the Bahamas, the British Virgin Islands, the Cayman Islands, Dominica, Grenada, Montserrat,  St. Kitts & Nevis, St. Lucia, and St. Vincent & the Grenadines.
Other than the United States, the US gallon is used in Liberia, Belize, Colombia, The Dominican Republic, Ecuador, El Salvador, Guatemala, Haiti, Honduras, Nicaragua, and Peru, but only for the sale of gasoline. All other products are sold in litres and its multiples and submultiples.
In the Turks & Caicos Islands, both the U.S. gallon and Imperial gallon are used, due to an increase in tax duties disguised by levying the same duty on the 3.79 L U.S. gallon as was previously levied on the 4.55 L Imperial gallon.
Relationship to other unitsEdit
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/ of an imperial pint or 1/ of an imperial gallon, while a US fluid ounce is 1/ of a US pint or 1/ 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.
Historically, a common bottle size for distilled spirits in the US was the "fifth", that is, one-fifth of a US gallon. Spirit sales in the US were switched to metric measures beginning in 1976; a 750 ml bottle is still sometimes known as a "fifth".
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 suggests 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 larger 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/ inches in diameter and 8 inches in depth. That made the dry gallon (9 1/)2 × π cubic inches ≈ 268.80252 in3. 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 in3 (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 6 inches deep and 7 inches in diameter, i.e. 6 in × (3 1/ 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, which is the result of the earlier definition with π approximated to 22/. 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 (the original "brass" was refined as the density of brass alloys vary depending on metallurgical composition). This was calculated as 4.546091879 L to ten significant figures. The precise definition of exactly 4.54609 cubic decimetres (also 4.54609 L after the litre was redefined in 1964, ≈ 277.419433 in3) was adopted shortly afterwards in Canada, and in 1976 was adopted in the United Kingdom.
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 historic gallons|
(gallons per cubic foot)
@ 62 °F)
|(cu in)||(L or dm3)||Diameter
|216 (Roman unciae)||≈ 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 (UK 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/ corn gallons||6.23||10|
|277.274||4.543460||Imperial Gallon (1824) as originally evaluated.||6.23||10|
|≈277.419433||4.54609||standard imperial gallon (metric) (1964 Canada gallon, 1976 UK gallon)||≈ 6.23||10||5⅔||11||0.000 2|
|≈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|
- IEEE Std 260.1-2014
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Before that date (November 1976) the definition in the Weights and Measures Act 1963 was such that the gallon could be calculated to be 4.546 091 879 dm3 to ten significant figures ... The return, in November 1976, by precise definition to what had earlier been used as an approximation for the value of the gallon (i.e. 4.546 09 dm3)...
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