A British Imperial unit of volume defined as the volume occupied by exactly ten pounds of distilled water of density 0.998 859 grams per millilitre in air of density 0.001 217 grams per millilitre. One gallon is therefore equal to 4.546 09 litres. In the US Customary system, one gallon is equal to 0.832 68 Imperial gallons or 3.785 44 litres. It is the volume occupied by 8.3359 pounds of distilled water and owes its origin to the Winchester or wine gallon.