Gallon

The gallon is a measure of volume approximately equal to four litres. Historically it has had many different definitions, but there are three definitions in current use. In United States customary units there are the liquid (≈ 3.79 L) and the lesser used dry (≈ 4.4 L) gallons. There is also the imperial gallon (≈ 4.55 L) which is in unofficial use within the United Kingdom and Ireland and is in semi-official use within Canada.^[1] The gallon, be it the imperial or U.S. gallon, 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,^[2] and is equal to exactly 3.785411784 litres or about 0.133680555 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 4.40488377086 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.^[2]
• The imperial (UK) gallon was legally defined as 4.54609 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.

On 1 January 2000, it ceased to be a legal unit of measure within the United Kingdom for economic, health, safety or administrative purposes.^[3] In 2005 a major step in metrication i.e. kilometres and litres, was taken in Ireland, only excluding draught beer.^[4]

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.^[5]

The Imperial gallon is used colloquially (and in advertising) in the United Kingdom and Canada for the fuel economy figures, in miles per gallon (elsewhere in Europe, the effective fuel consumption is advertised in litres per 100 km. Canadian and US web sites, as well as some Pacific or Asian web sites some times use the MPG style "km per litre", which is not used in Europe. The gallon continues to be used as a unit of measure for fuel^[5] in Antigua and Barbuda,^[6] Belize,^[7][8] Burma (Myanmar),^[9][10][11] Cayman Islands, Grenada,^[12][13] Guyana and Sierra Leone. The United Arab Emirates switched from Imperial gallons to litres on 1 January 2010.

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. An Imperial fluid ounce is 4% smaller than a U.S. 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 U.S. citizens referred to the imperial fluid measure as a "five quart gallon", since an imperial gallon has a volume equal to 4.804 U.S. 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 268.8 cu in (≈ 4.405 L),
• The wine gallon, or Queen Anne's gallon, which was 231 cu in^[14] (≈ 3.79 L), and
• The ale gallon of 282 cu in (≈ 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 cu in ≈ 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 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 × (3 ¹⁄₂)² × π ≈ 230.90706 cu in. It had been redefined during the reign of Queen Anne, in 1706, as 231 cu in 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 gallons was (9 ¹⁄₄)²:6 × (3 ¹⁄₂)² = 1369:1176, 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 1.164115646 slightly changed to 1.163647186 with current definitions (268.8025:231 = 107521:92400 ≈ 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 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.

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.

Abbreviations

Gallons per minute is sometimes abbreviated as GPM. Million gallons per year is sometimes abbreviated as mgy, MGY, mmgy, MMgy, or MMGY.

Examples of gallons

See also

• Comparison of the imperial and US customary measurement systems
• English units of wine casks

References

External links

• Oil Measurements and Conversions