A gallon's a measure of volume of approximately four litres. Historically it's had many different definitions, but there're three definitions in current use. These are the U.S. liquid gallon (≈ 3.8 litres) and the U.S. dry gallon (≈ 4.4 L) which are in use in the United States, and the Imperial (UK) gallon (≈ 4.5 L) which's in unofficial use within the United Kingdom. One or other version's sometimes found in other English-speaking countries. Definitions The U.S. liquid gallo… (More on Gallon)