Gallon

The gallon is a unit of volume in British imperial units and United States customary units.

The imperial gallon (imp gal) is defined as 4.54609 litres, and is or was used in the United Kingdom and its former colonies, including Ireland, Canada, Australia, New Zealand, India, South Africa, Malaysia and some Caribbean countries, while the US gallon (US gal) is defined as 231 cubic inches (3.785411784 L), and is used in the United States and some Latin American and Caribbean countries.

There are four gills in a pint, two pints in a quart, and four quarts (quarter gallons) in a gallon, with the imperial gill being divided into five imperial fluid ounces and the US gill being divided into four US fluid ounces: this, and a slight difference in the sizes of the imperial fluid ounce and the US fluid ounce, give different sizes for the imperial gallon and US gallon.

The IEEE standard symbol for both the imperial and US gallons is gal, not to be confused with the gal (symbol: Gal), a CGS unit of acceleration.

The gallon currently has two definitions, in the imperial system and in the US customary system.

Historically, there were many definitions and redefinitions: see § Sizes of gallons for details.

The British imperial gallon (frequently called simply "gallon") is defined as exactly 4.54609 litres. It is used in Britain and some other Commonwealth countries, and until 1976 was defined as the volume of water at 62 °F (16.7 °C) whose mass is 10 pounds (4.5359237 kg).

There are four imperial quarts in a gallon, two imperial pints in a quart, and 20 imperial fluid ounces in an imperial pint, making an imperial fluid ounce ⁠1/160⁠ of an imperial gallon.