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 foot. 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).
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