gallon

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 litres) and the U.S. dry gallon (≈ 4.4 L) which are in use in the United States, and the Imperial (UK) gallon (≈ 4.5 L) which is in unofficial use within the United Kingdom. One or other version is sometimes found in other English-speaking countries.

Definitions

A one U.S. gallon gas can purchased near the U.S.-Canada border. It shows equivalences in Imperial gallons and litres.

• The U.S. liquid gallon is legally defined as 231 cubic inches,^[1] and is equal to exactly 3.785 411 784 litres (1 L = 10⁻³ m³) or 0.133 680 555… cubic feet. This is the most common definition of a gallon in the United States. The U.S. fluid ounce is defined as 1/128 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 4.404 883 770 86 litres. 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.^[1]
• The Imperial (UK) gallon was legally defined as 4.546 09 litres. This definition is used in some Commonwealth countries and Ireland, and is based on the volume of 10 pounds 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 1/160 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^[2]. In 2005 a major step in metrication i.e. kilometres and litres, was taken in Ireland, only exluding draught beer^[3].

Worldwide usage of gallons

As of 2005 the U.S. liquid gallon continued to be used as a unit of measure for fuel in Colombia, Dominican Republic, Ecuador, El Salvador, Guatemala, Haiti, Honduras, Liberia, Nicaragua, Panama, Peru, and the United States.^[4]

The Imperial gallon is used colloquially (and in advertising) in the United Kingdom 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^[4] in Antigua and Barbuda,^[5] Belize,^[6][7] Burma (Myanmar),^[8][9][10] Cayman Islands, Grenada,^[11][12] Guyana, Sierra Leone, and the United Arab Emirates.

The word has also been used as translation for several foreign units of the same magnitude.^{[citation needed]}

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.

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 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 18½ inches in diameter and 8 inches depth. That made the dry gallon 9¼²·π in³ ≈ 268.80252 cubic inches. 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 268.8025 cubic inches exactly (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 is 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. 6·3½²·π ≈ 230.90706 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 U.S. 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 1.164115646 slightly changed to 1.163647186 with current definitions (268.8025:231 = 107521:92400 ≈ 1351:1161). In some contexts it is or was necessary to disambiguate between those two U.S. gallons, so “liquid” or “fluid” and “dry” respectively are then added to the name.

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 grams per millilitre 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, ca. 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.

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

References

See also

• Comparison of the imperial and US customary measurement systems

External links

Look up Gallon in Wiktionary, the free dictionary.

• Oil Measurements and Conversions