Gallon

From Wikipedia, the free encyclopedia.

The gallon (abbr. gal) is an English unit of volume used for measuring liquids (as well as dry matter), with varying definitions between 3½ and 4¾ litres (l). The word has also been used as translation for several foreign units of the same magnitude.

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Today

An Imperial gallon is exactly 4.54609 litres.
A US (liquid) gallon is exactly 231 in³ (3.785411784 L).

The ratio between them is approximately 6:5.

Both these gallons in current use—differentiated if necessary by a prefix “US” or “Imp.”—are subdivided into eight pints, but the US pint is further subdivided into 16 ounces (fl oz), whereas the Imperial one holds 20. Therefore a US fl oz is ca. 29.6 ml and an Imp. fl oz is ca. 28.4 ml. Thus the US fluid ounce and all its subdivisions are bigger than their Imperial equivalents, but all the other, larger US liquid measures, including the gallon, are smaller than their Imperial counterparts. See also: Comparison at English units article

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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 268.8 in³ (4.405 l),
the wine gallon, or “Queen Anne’s gallon”, which was 231 in³ (3.79 l), and
the ale gallon of 282 in³ (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 18½ inches in diameter and 8 inches depth. That made the dry gallon 9¼²·π in³ = 268.80252 in³. The bushel, which like dry quart and pint still sees some use, was later defined to be 2150.42 in³ exactly, making its gallon 268.8025 in³ exactly (4.404 842 803 2 l). In previous centuries there had been a corn gallon of around 271 to 272 cubic inches, too.

The wine, fluid or liquid gallon is the standard US gallon since the early 19th century. The wine gallon, which some sources relate to the volume occupied by eight mediæval 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. 6·3½²·π = 230.90706 in³. It had been redefined during the reign of Queen Anne, in 1706, as 231 in³ exactly (3 × 7 × 11 in³), 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 in³) was actually in use, so this statute became necessary. It remains the US definition today.

The original ratio between corn and wine gallon is 9¼²:6·3½² = 1369:1176, but 268.8:231 is exactly 64:55 or ca. 13:11. This approximation is still applicable, although the ratio of 1.164 115 646 slightly changed to 1.163 647 186 with current definitions (268.8025:231 = 107521:92400 ~= 1344:1165). In some contexts it is or was necessary to disambiguate between those two US gallons, so “liquid” or “fluid” and “dry” respectively are then added to the name.

In 1824, Great 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 lb. 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 lb of distilled water of density 0.998 859 g/mL weighed in air of density 0.001 217 g/mL against weights of density 8.136 g/mL. This works out at approximately 4.546 090 3 L (277.441 6 in³). The metric definition of exactly 4.546 09 dm³ (also 4.546 09 L after the litre was redefined in 1964, ca. 277.419 433 in³) was adopted shortly afterward in Canada; for several years, the conventional value of 4.546 092 L was used in the UK, until the Canadian convention was adopted in 1985.

Before and into the 19th century there were also several other gallons in use. Examples: