Gallons - Background & History
The US Gallon
The US gallon, when referring to liquids, is defined as 231 cubic inches or 0.133680555 cubic feet. This is 3.785411784 liters. (There is also a US dry gallon of 268.8025 cubic inches or one eighth of a Winchester bushel, although this is not in common usage.)
The US gallon continues to be used as a measure for gasoline in Colombia, Dominican Republic, Ecuador, El Salvador, Guatemala, Haiti, Honduras, Liberia, Nicaragua, Panama, Peru, and the United States.
The Imperial Gallon (also known as the UK Gallon)
The imperial gallon was legally defined as 4.54609 litres and is based on the volume of 10lbs of water at 62 degree fahrenheit.
The imperial gallon is used as a measure of petrol in Antigua and Barbuda, Belize, Cayman Islands, Grenada, Guyana, Myanmar, Sierra Leone, and the United Arab Emirates. It is also commonly used in advertising of cars with a miles per gallon figure in both the UK and Canada, although it has ceased to be used as a legal trading measure in the UK. (Beer and milk are both still sold in pints in the UK although packaging shows a value in milliliters.)
The gallon traces its history back to the Roman congius, a measure of liquid which contained six sextarii or the 8th part of the amphora. This measure was taken to and used in England and appears in a charter of Edmund I in 946AD.
The gallon although roughly based on the congius developed over time and cme to mean different amounts dependent upon what was being measured, however following various laws and statutes being passed the gallon once again became standardized and by the late 18th century there were just three gallons in common usage, the corn gallon (268.8 cubic inches), the ale gallon (282 cubic inches) and the wine gallon (231 cubic inches). In the early nineteenth century the US adopted the wine gallon as a its single standard gallon and the UK (1824) adopted the ale gallon as its standard gallon, which is where the difference between the US and the imperial gallon originates.