Gallon
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The gallon is a unit of volume used for measuring liquids (as well as dry matter).
- An Imperial gallon is exactly 4.546 09 litres: see Imperial unit.
- In the United States a fluid gallon is exactly 3.785 411 784 litres,
- a dry gallon is exactly 4.404 842 803 2 litres: see U.S. customary units.
- The U.S. liquid gallon is abbreviated U.S. gal whereas the Imperial gallon is the Imp. gal.
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 in³ (and became the legal basis for the U.S. fluid gallon), and the ale gallon, of 282 in³.
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 lb of distilled water weighed in air with brass weights with the barometer standing at 30 in and at a temperature of 62 °F. In 1963, this definition was refined as the space occupied by 10 lb 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 in³. 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 in long and 7 in in diameter, or 230.907 in³. 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 fl oz whereas an Imperial pint is 20 fl oz. 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 in³
- standard wine gallon preserved at Guildhall
- 231 in³
- statute of 5th of Anne
- 264.8 in³
- ancient Rumford quart (1228)
- 265.5 in³
- Exchequer (Henry VII., 1091, with rim)
- 266.25 in³
- ancient Rumford (1228)
- 268.75 in³
- Winchester, statute 13 + 14 by William III.
- 271 in³ − 2 spoonfuls
- Exchequer (Henry VII., 1601, E.E.)
- 271 in³
- Exchequer (1601, E.), corn
- 272 in³
- corn (1688)
- 277.18 in³
- coal, statute 12 of Anne
- 278 in³
- Exchequer (Henry VII., with copper rim)
- 278.4 in³
- Exchequer (1601 and 1602 pints)
- 280 in³
- Exchequer (1601 quart)
- 282 in³
- Treasury (gallon for beer and ale)da:Gallon
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