The gallon is a unit of volume used for measuring liquids.

• In the US a gallon is 3.78541 litre: see U.S. customary units
• An Imperial gallon is 4.54609 litre: 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, which was 231 cubic inches, and the ale gallon, of 282 cubic inches.

In 1824, Britain adopted an a close approximation to the ale gallon known as the Imperial gallon. The Imperial gallon is based on the volume of 10 pounds of distilled water weighed in air with the barometer standing at 30 inches and at a temperature of 62° Fahrenheit (which works out at 277.41945 cubic inches).

The United States by this time, had already standardised on a close approximation to 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. Today, however, the gallon is 231 in³ exactly. Thus 10 US gallons equals 8.327 Imperial gallons. The Imperial gallon is about a fifth larger than the US gallon.

Both the Imperial and United States gallon are equal to 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 but 160 fl. oz. This means that A US fluid ounce is around 1.8047 cu. ins and an Imperial fl. oz is around 1.7339 cu. ins. So the US fluid ounce is actually bigger than the imperial, although the US gallon is smaller.