A measure of water volume, principally used in the western states of the US. An acre-foot (acre-ft) is the volume of water required to cover 1 acre of land (43,560 square feet) to a depth of 1 foot. Equal to 325,851 gallons or 1,233 cubic meters. As a very rough estimate, an acre-foot is often used as a measure of how much water a (western US) family of four might use in one year.