The gallon is a unit of volume used for measuring liquids (as well as dry matter).

• In the US a fluid gallon is exactly 3.785 411 784 litres: see U.S. customary units
• An Imperial gallon is exactly 4.546 09 litres: see Imperial unit.

At one time, the volume of a gallon depended on what you were measuring, and where you were measuring it. But, by the 19th century, two definitions were in common use. The wine gallon, or "Queen Anne's gallon", which was 231 cubic inches (and became the legal basis for the U.S. fluid gallon), and the ale gallon, of 282 cubic inches.

In 1824, Britain adopted a close approximation to the ale gallon known as the Imperial gallon. The Imperial gallon was based on the volume of 10 pounds of distilled water weighed in air with brass weights with the barometer standing at 30 inches and at a temperature of 62 degrees Fahrenheit. In 1963, this definition was refined as the space occupied by 10 pounds of distilled water of density 0.998 859 g/mL weighed in air of density 0.001 217 g/mL against weights of density 8.136 g/mL. This works out at approximately 277.419 45 cubic inches. The metric definition of exactly 4.546 09 L was adopted shortly afterward.

The United States, by this time, had already standardised on the old wine gallon. It was at one time defined as the volume of a cylinder 6 inches long and 7 inches in diameter, or 230.907 cubic inches. It had been redefined during the reign of Queen Anne as 231 in³ exactly, which remains the U.S. definition today. Thus 10 U.S. gallons equals approximately 8.327 Imperial gallons. The Imperial gallon is about a fifth larger than the U.S. gallon.

Both the Imperial and United States gallon are divided into 8 pints. However in the US a pint is 16 fluid ounces whereas an Imperial pint is 20 fluid ounces. Thus a U.S. gallon is 128 fl oz and an Imperial gallon is 160 fl oz; this means that a US fluid ounce is around 1.8047 in³ and an Imperial fl oz is around 1.7339 in³. The US fluid ounce is actually bigger than the imperial, although the US gallon is smaller.

History

Until and before the 19th century there were even more gallons in use. Examples:

224 in.³ 

standard wine gallon preserved at Guildhall

231 

statute of 5th of Anne

264.8 

ancient Rumford quart (1228)

265.5 

Exchequer (Henry VII., 1091, with rim)

266.25 

ancient Rumford (1228)

268.75 

Winchester, statute 13 + 14 by William III.

271 − 2 spoonfuls 

Exchequer (Henry VII., 1601, E.E.)

271 

Exchequer (1601, E.), corn

272 

corn (1688)

277.18 

coal, statute 12 of Anne

278 

Exchequer (Henry VII., with copper rim)

278.4 

Exchequer (1601 and 1602 pints)

280 

Exchequer (1601 quart)

282 

Treasury (gallon for beer and ale)