http://en.wikipedia.org/wiki/Gallon
The gallon (abbr. gal) is a English unit of volume used for measuring liquids (as well as dry matter), with varying definitions between 3½ and 4¾ litres. The word has also been used as translation for several foreign units of the same magnitude.
An Imperial gallon is exactly 4.546 09 litres.
In the United States customary system,
a fluid gallon is exactly 231 in³ (3.785 411 784 litres) and
a dry gallon is exactly 268.8025 in³ (4.404 842 803 2 litres).
The ratios between them are approximately 66:55:64.
To disambiguate, 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, three definitions were in common use. The wine gallon, or "Queen Anne's gallon", which was 231 in³, and the ale gallon, of 282 in³ (4.62 L). Grain and other dry commodities were measured by the corn gallon, which is one eighth of the Winchester bushel, a measure of 18½ inches in diameter and 8 inches depth (now 2150.42 in³ exactly). That made the dry gallon 9¼²·π in³, i.e. about 268.8 in³.
In 1824, Britain adopted a close approximation to the ale gallon known as the Imperial gallon. Inspired by the kilogram–litre relationship, 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 inches of mercury 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 4.546 090 3 L; (277.441 6 in³). The metric definition of exactly 4.546 09 dm³ (also 4.546 09 L after the litre was redefined in 1964, ca. 277.419 433 in³) was adopted shortly afterward in Canada; for several years, the conventional value of 4.546 092 L was used in the U.K., until the Canadian convention was adopted in the 1990s.
The United States, earlier in the 19th century, 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, in the early 18th century, as 231 in³ exactly (3 × 7 × 11 in³), which is the result of the earlier definition with π approximated to 22/7. This 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, i.e. 5 Imp. gal ≈ 6 U.S. gal.
Both the Imperial and United States gallon are divided into 8 pints. However in the US a pint is 16 fluid ounces (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.