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

• The U.S. liquid gallon is legally defined as 231 cubic inches,Authorized tables, U.S. Code, Title 15, ch. 6, subchapter I, sec. 205, accessed 19 July 2008. and is equal to exactly nowrap: 3.785 411 784 litres (1 L = 10⁻³ m³) or nowrap: 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 nowrap: 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.
• The Imperial (UK) gallon was legally defined as nowrap: 4.546 09 litres. This definition is used in 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 from 1st January 2000 it ceased to be legal within the United Kingdom for economic, health, safety or administrative purposes.

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.

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 in Antigua and Barbuda, Belize, Burma (Myanmar), Cayman Islands, Grenada, Guyana, Sierra Leone, and the United Arab Emirates.