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There are three definitions in current use:

• U.S. liquid gallon is legally defined as 231 cubic inches, and is equal to 3.785411784 litres (exactly) or about 0.13368 cubic feet. This is the most common definition of a gallon in the USA. The U.S. fluid ounce is defined as 1/128 of a U.S. gallon.
• U.S. dry gallon is one-eighth of a U.S. Winchester bushel of 2150.42 cubic inches, thus 268.8025 cubic inches (exactly) or 4.40488377086 litres (exactly)
• Imperial (UK) gallon is legally defined as 4.54609 litres (~277.42 cu in), which is about 1.2 U.S. liquid gallons. This definition is used in the United Kingdom, and is based on the volume of 10 pounds of water at 62 °F. (A U.S. liquid gallon weighs about 8.33 pounds at the same temperature.) The Imperial fluid ounce is defined as 1/160 of an Imperial gallon. The Imperial gallon is no longer legal, in the UK, for trade or public administration purposes, but it is used colloquially (and in advertising) for fuel consumption figures in miles per gallon. The Imperial gallon continues to be used as a unit of measure for fuel in various countries (for example, United Arab Emirates and Sierra Leone).

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

The U.S. dry gallon is less commonly used.

The gallons in current use are subdivided into eight pints or four quarts. The liquid gallons are also subdivided into 32 gills.

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 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 in × 7 in × 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 ~= 1344:1165). 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.

• Comparison of the imperial and US customary measurement systems