Gallon

The gallon is a volume unit used in both the British imperial system and the US customary system. Over time, it has taken many forms, such as the corn gallon, wine gallon, and ale gallon, each with its own definition.

Today, there are two main definitions. The imperial gallon (imp gal) equals exactly 4.54609 litres. It was once standard in the United Kingdom, Ireland, Canada, Australia, New Zealand, India, South Africa, Malaysia, and several Caribbean nations, and is still used in some places. The US gallon (US gal) is defined as 231 cubic inches, or 3.785411784 litres, and is used in the United States, parts of Latin America, and some Caribbean countries.

Both gallons are split into four quarts, each quart into two pints, and each pint into two cups (though cups are not common outside the US). Cups are further divided into two gills. However, the imperial gill contains five fluid ounces, while the US gill contains four. This difference, along with a slight variation in the size of the fluid ounce itself, means an imperial fluid ounce is 1/160 of an imperial gallon, while a US fluid ounce is 1/128 of a US gallon. As a result, one imperial gallon equals about 1.20095 US gallons.

The imperial gallon was originally defined as the volume of water at 62 °F (16.7 °C) that weighs 10 pounds (4.5359237 kg). This definition was used until 1976. Today, it is no longer legal to sell goods by the imperial gallon in the UK and Ireland, though it can still be used as a secondary unit. The US gallon is legally defined as exactly 231 cubic inches. At 3.983 °C (39.169 °F), a US gallon of water weighs 3.785411784 kg (8.3454 lb), making it about 83.27% of an imperial gallon. To account for temperature-related expansion and contraction in trade, materials like petroleum and alcohol are often measured at a reference temperature of 60 °F (15.6 °C).

The imperial gallon remains the standard for petrol in four British Overseas Territories (Anguilla, British Virgin Islands, Cayman Islands, Montserrat) and four countries (Antigua and Barbuda, Grenada, Saint Lucia, Saint Vincent and the Grenadines). The United Arab Emirates switched to litres for petrol in 2010, Guyana in 2013, and Myanmar in 2014. Dubai’s water billing moved from imperial gallons to cubic metres in March 2025. Antigua and Barbuda has proposed switching petrol to litres since 2015. In the European Union, the gallon was removed from the list of primary trade units in 1994, though it can still appear as a supplementary unit. Ireland and the UK passed laws replacing it with the litre as the primary trade unit in 1993 and 1995, respectively. Miles per imperial gallon is still the main fuel economy measure in the UK and a supplementary one in Canada.

The US gallon is used for petrol in four US territories (American Samoa, Northern Mariana Islands, Guam, US Virgin Islands) and ten countries: Colombia, Ecuador, Guatemala, Haiti, Peru, the United States, the Marshall Islands, Micronesia, Palau, and Liberia. Both the imperial and US gallons are used in the Turks and Caicos Islands (due to a tax change that applied the same duty to the smaller US gallon) and the Bahamas.

In some Middle Eastern countries, such as the United Arab Emirates and Bahrain, 18.9-litre water cooler bottles are sold as “five-gallon” bottles. The IEEE standard symbol for both gallons is “gal,” which should not be confused with “Gal,” the symbol for a CGS unit of acceleration.