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A gallon is a measure of volume of approximately four litres. Historically it has had many different definitions, but there are three definitions in current use. These are the U.S. liquid gallon (≈ 3.8 L) and the lesser used U.S. dry gallon (≈ 4.4 L) which are in use in the United Statesmarker, and the Imperial gallon (≈ 4.5 L) which is in unofficial use within the United Kingdommarker and Irelandmarker and is in semi-official use within Canadamarker (See Canadian units). The gallon, be it the Imperial or U.S. gallon, is sometimes found in other English-speaking countries.

Definitions

  • The U.S. liquid gallon is legally defined as 231 cubic inches, and is equal to exactly or about . This is the most common definition of a gallon in the United States. The U.S. fluid ounce is defined as of a U.S. gallon.
  • The U.S. dry gallon is one-eighth of a U.S. Winchester bushel of 2150.42 cubic inches, thus it is equal to exactly 268.8025 cubic inches or . The U.S. dry gallon is less commonly used, and is not listed in the relevant statute, which jumps from the dry quart to the peck.
  • The imperial (UK) gallon was legally defined as . This definition is used in some Commonwealth countries and Irelandmarker, and is based on the volume of 10 pound of water at 62 °F. (A U.S. liquid gallon of water weighs about 8.33 pounds at the same temperature.) The imperial fluid ounce is defined as of an imperial gallon.
As of 1 January 2000 it ceased to be legal within the United Kingdom for economic, health, safety or administrative purposes.In 2005 a major step in metrication i.e. kilometres and litres, was taken in Ireland, only excluding draught beer.

Worldwide usage of gallons

As of 2005 the U.S. liquid gallon continued to be used as a unit of measure for fuel in Colombiamarker, Dominican Republicmarker, Ecuadormarker, El Salvadormarker, Guatemalamarker, Haitimarker, Hondurasmarker, Liberiamarker, Nicaraguamarker, Panamamarker, Perumarker, and the United Statesmarker.

The Imperial gallon is used colloquially (and in advertising) in the United Kingdommarker and Canadamarker for the fuel economy figures, in miles per gallon (elsewhere in Europe, the effective fuel consumption is often advertised in litres per 100 km, or km per litre). It continues to be used as a unit of measure for fuel inAntigua and Barbudamarker, Belizemarker, Burmamarker (Myanmar), Cayman Islandsmarker, Grenadamarker, Guyanamarker, Sierra Leonemarker,and the United Arab Emiratesmarker.

The word has also been used as translation for several foreign units of the same magnitude.

Subdivisions

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 U.S. units, differ in volume and are therefore not interchangeable. The principal difference is that the Imperial pint contains 20 Imperial fluid ounces, whereas the U.S. pint contains 16 U.S. fluid ounces. A U.S. fluid ounce is approximately 4% bigger than an Imperial fluid ounce and therefore they are often used interchangeably, whereas U.S. and Imperial pints and gallons are sufficiently different that they should not be used interchangeably, although they often are.

Earlier in the 20th century, while Canada was still using the imperial gallon as its automotive fuel measurement, some US citizens referred to the imperial fluid measure as a "five quart gallon", since an imperial gallon has a volume equal to 4.804 US quarts.

History

At one time, the volume of a gallon depended on what was being measured, and where it was being measured. But, by the end of the 18th century, three definitions were in common use:
  • The corn gallon, or Winchester gallon, of about ,
  • The wine gallon, or Queen Anne’s gallon, which was , and
  • The ale gallon of .


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 bushel, originally a cylindrical measure of inches in diameter and 8 inches in depth. That made the dry gallon . The bushel, which like dry quart and pint still sees some use, was later defined to be 2150.42 in3 exactly, making its gallon exactly ( ).In previous centuries, there had been a corn gallon of around 271 to 272 in3.

The wine, fluid, or liquid gallon has been the standard U.S. 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. . It had been redefined during the reign of Queen Anne, in 1706, as exactly , which is the result of the earlier definition with π approximated to . 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 was actually in use, so this statute became necessary. It remains the U.S. definition today.

The original ratio between corn and wine gallons was , but 268.8:231 (i.e. the ratio between the rounded quantities, in cubic inches) is exactly 64:55 or approximately 13:11. This approximation is still applicable, although the ratio of slightly changed to with current definitions (268.8025:231 = : ≈ 1351:1161). In some contexts, it was necessary to disambiguate between those two U.S. gallons, so “liquid” or “fluid” and “dry” respectively were added to the names.

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 . In 1963, this definition was refined as the space occupied by 10 pounds of distilled water of density weighed in air of density against weights of density . This works out at approximately ( ). The metric definition of exactly cubic decimetres (also after the litre was redefined in 1964, ≈  ) was adopted shortly afterwards in Canada; for several years, the conventional value of was used in the United Kingdom, until the Canadian convention was adopted in 1985.

Before and into the 19th century there were also several other gallons in use, with varying definitions. These are summarized in the table below. During some eras, the gallon was based on an exact conversion with a linear measure cubed. Other eras, the gallon was based on a rational approximation to the volume of a cylinder that could be used as a standard container, such as a basket, barrel, or jar. Other definitions were based on the density of a commodity, occasionally water, but more often a more marketable good such as wine or oats. Given these options and the variety of cultures that have used the gallon, it is not surprising that the exact value has drifted over the centuries.

Examples of gallons

Volume Definition Inverted volume

(gallons per cubic foot)
Approx.

mass of

water (pounds

per gallon

@ 62 °F)
Cylindrical approximation
(in3) (L or dm3) Diameter

(in)
Height

(in)
Relative

error
(%)
216 Roman congius 8 7.8 5 11 0.01
224 ≈ 3.6707 preserved at the Guildhall, Londonmarker (old UK wine gallon) 7.71 8.09 9 3.5 0.6
231 3.785411784 statute of 5th of Queen Anne (U.S. wine gallon, standard U.S. 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 Winchester, statute 13 + 14 by William III (corn gallon, old U.S. dry gallon) 6.43 9.71 18.5 1
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) 6.23 10      
(ca.) standard Imperial gallon (metric) (1964 Canada gallon, 1985 UK gallon)          
Imperial gallon (1824) (traditional UK ale gallon) 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      


See also



References

  1. Authorized tables, U.S. Code, Title 15, ch. 6, subchapter I, sec. 205, accessed 19 July 2008.


External links




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