A US Liquid gallon is a measure of volume equaling 231 cubic inches.
An Imperial gallon is defined as 4.54609188 litres. It was originally defined as the volume of 10 pounds of water at 62 degrees farenheight. This unit is used in England and some parts of the British Commonwealth.
A US gallon is smaller than the Imperial gallon, by about 20%. The American gallon is 3.78541178 liters.