Where does the word gallon come from?

In the Roman period the term gallon was used to represent the general idea a capacity or volume of material, and seemed to have been applied to both wet and dry measures. The earliest official attempt to standardize the gallon was during the thirteenth century when Edward I set a standard of eight pounds of wheat; but gallons with eight pounds dry weight (Avoirdupois) and others with eight pounds liquid (Troy weight) coexisted with others that seemed to meet no particular standard at all. In 1707, during the reign of Queen Anne, a new standard was written. The English established a Wine Gallon, which was also called the Queen Anne gallon and was defined to be 221 c.c. and was about the size of the traditional wine gallon in England. This is the size of the US Gallon to this day. A beer gallon also had become more or less standardized by tradition at ten pounds of beer (or ale).

This was very close to the dry weight of wheat in the same container and in 1824 the English established a liquid measure, the Imperial Gallon with a capacity to hold 10 pounds (Avoirdupois) of distilled water . Because this was after the US became independent of Great Britain, the two countries have since had different units for a liquid gallon. The US liquid gallon holds about 8.3 pounds of water. The "dry gallon" is seldom used in the US.