An acre-foot is a volume occupied by one acre of land with a depth of one foot. Volume for an acre-foot is equivalent to 43,560 cubic feet (An acre measures 43,560 square feet). Acre-feet is commonly used as an irrigation measurement; it represents the amount of water needed to cover one acre of land to a depth of one foot. Approximately 325,851 gallons of water would fill an acre of land to a depth of one foot. An acre-foot is a useful measurement and is commonly used in the United States for large-scale water resources such as canals, rivers, reservoirs, and aqueducts. It more convenient to measure the volume of these large spaces in acre-feet rather than gallons or liters, which is the standard unit of volume measurement in the International System of Units.