The gallon is a unit of volume in British imperial units and United States customary units.
The imperial gallon is defined as 4.54609 litres, and is or was used in the United Kingdom and its former colonies, including Ireland, Canada, Australia, New Zealand, India, South Africa, Malaysia and some Caribbean countries, while the US gallon is defined as 231 cubic inches, and is used in the United States and some Latin American and Caribbean countries. From Wikipedia
