The gallon is a measure of volume. Historically it has had many different definitions, but there are three definitions in current use. There is the imperial gallon (≈ 4.546 l) which is used in the United Kingdom and is in semi-official use within Canada. In United States customary units there are the liquid (≈ 3.79 l) and the lesser used dry gallons (≈ 4.40 l). The gallon, be it the imperial or US gallon, is sometimes found in other English-speaking countries.
- The imperial (UK) gallon was legally defined as 4.54609 L. This definition is used in some Commonwealth countries, and is based on the volume of 10 pounds of water at 62 °F. (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. On 1 January 2000, it ceased to be a legal unit of measure within the United Kingdom for economic, health, safety or administrative purposes.
- The US liquid gallon is legally defined as 231 cubic inches, and is equal to exactly 3.785411784 litres or about 0.133680556 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 gallon.
- The US dry 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 colloquially (and in advertising) in Canada, and used in the United Kingdom for fuel economy figures (i.e. miles per gallon)—elsewhere in Europe, the effective fuel consumption is advertised in litres per 100 km. The Imperial gallon also continues to be used as a unit of measure for fuel in Antigua and Barbuda, Belize, Burma (Myanmar), Cayman Islands, Grenada, and Guyana. The United Arab Emirates switched from imperial gallons to litres on 1 January 2010. Sierra Leone switched to selling fuel by the litre in May 2011.. Antigua and Barbuda will convert to litres before 2015.
Relationship to other units
The gallons in current use are subdivided into eight pints or four quarts. Pints are further subdivided into fluid ounces and liquid gallons are also subdivided into 32 gills, i.e. a quarter of a pint. The sub-units of pint and fluid ounce, 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 U.S. 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.
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. The only vestige of this system still operating in the United Kingdom is that draught beer and cider must still be sold in thirds of UK pints, halves of UK pints, or full UK pints (and multiples thereof). Wine in the United Kingdom has been sold in metric quantities since the early 1990s.
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 × (3 1⁄2)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 (277.4416 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; for several years, the conventional value of 4.546092 L was used in the United Kingdom, until the Canadian convention was adopted in 1985.
Comparison of gallons
Volume Definition Inverted volume
(gallons per cubic foot)
@ 62 °F)
Cylindrical approximation (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, 1091, 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 ≈ 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
- Comparison of the imperial and US customary measurement systems
- English units of wine casks
- English units of brewery casks
- ^ 
- ^ http://laws-lois.justice.gc.ca/eng/acts/W-6/page-19.html
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- ^ "Burma's Activists March against Fuel Price". HikeThe Irrawaddy News Magazine Online Edition Covering Burma. 2007-08-20. http://www.irrawaddy.org/article.php?art_id=8286. Retrieved 2008-01-16. "The government, which holds a monopoly on fuel sales and subsidizes them, raised prices of fuel from 1,500 kyats (US $1.16) to 3,000 kyats ($2.33) per imperial gallon for diesel and to 2,500 kyats ($1.94) for gasoline."
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- ^ "UAE to replace gallon with litre from January 1". gulfnews.com. 2009-12-14. http://gulfnews.com/news/gulf/uae/general/uae-to-replace-gallon-with-litre-from-january-1-1.553391. Retrieved 2011-10-24.
- ^ "Introduction of the Metric System and the Price of Petroleum Products". Sierra Leone Embassy, Washington. 2011-04-01. http://embassyofsierraleone.net/latest-embassy-news/188-introduction-of-the-metric-system-and-the-price-of-petroleum-products-. Retrieved 2011-10-24.
- ^ "Minister Lovell Addresses Metric Conversions". Caribarena Antigua. 2011-10-18. http://www.caribarena.com/antigua/news/economy/98673-minister-lovell-addresses-metric-conversions.html. Retrieved 2011-10-24.
- ^ http://www.sizes.com/units/gallon_english_wine.htm
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