Gallon

The gallon is a measure of liquid capacity in both the US customary units and the British imperial systems of measurement. Three significantly different sizes are in current use: the imperial gallon defined as 7000454609000000000♠4.54609 litres, which is used in the United Kingdom, Canada, and some Caribbean nations; the US gallon defined as 231 cubic inches (3.785 l), which is used in the US and some Latin American and Caribbean countries; and the least-used US dry gallon defined as ¹⁄₈ US bushel (4.405 l).

Definitions

The gallon currently has one definition in the imperial system, and two definitions (liquid and dry) in the US customary system. Historically, there were many definitions and redefinitions.

The imperial gallon

The imperial (UK) gallon, now defined as exactly 7000454609000000000♠4.54609 litres (about 277.42 cubic inches), is used in some Commonwealth countries and was originally based on the volume of 10 pounds (approximately 4.54 kg) of water at 62 °F (17 °C). The imperial fluid ounce is defined as ¹⁄₁₆₀ of an imperial gallon; there are four quarts in a gallon, two pints in a quart, and 20 Imperial fluid ounces in an imperial pint.

United States customary units

United States customary units are a system of measurements commonly used in the United States. The United States customary system (USCS or USC) developed from English units which were in use in the British Empire before American independence. However, the British system of measures was overhauled in 1824 to create the imperial system, changing the definitions of some units. Therefore, while many U.S. units are essentially similar to their Imperial counterparts, there are significant differences between the systems.

The majority of U.S. customary units were redefined in terms of the meter and the kilogram with the Mendenhall Order of 1893 and, in practice, for many years before. These definitions were refined by the international yard and pound agreement of 1959. Americans primarily use customary units in commercial activities, as well as for personal and social use. In science, medicine, many sectors of industry and some of government, metric units are used. The International System of Units (SI), the modern form of the metric system, is preferred for many uses by the U.S. National Institute of Standards and Technology (NIST).