A unit of volume often used in oil reservoir analysis, equivalent to the volume (as of oil or water) necessary to cover 1 acre to a depth of 1 foot. One of the most common units of measure used for reservoir capacity. In the U.S., the thickness of a pay zone is measured in feet, and the area of the reservoir is measured in acres. An acre-foot is a volume of reservoir rock that is one acre in area and one foot thick