The acre-foot is a non-SI unit of volume equal to about 1,233 m³ commonly used in the western United States in reference to large-scale water resources, such as reservoirs, aqueducts, canals, sewer flow capacity, irrigation water, and river flows.

An acre-foot equals the volume of water needed to fill approximately an eight-lane swimming pool, 82 ft (25 m) long, 52 ft (16 m) wide and 9.8 ft (3 m) deep.

As the name suggests, an acre-foot is defined as the volume of water that would cover one acre of surface area to a depth of one foot.

Since an acre is defined as a chain by a furlong (i.e. 66 ft × 660 ft or 20.12 m × 201.17 m), an acre-foot is 43,560 cubic feet (1,233.5 m³).

There have been two definitions of the acre-foot (differing by about 0.0006%), using either the international foot (0.3048 m) or a U.S. survey foot (exactly ⁠1200/3937⁠ meters since 1893). On December 31, 2022, the National Institute of Standards and Technology, the National Geodetic Survey, and the United States Department of Commerce deprecated use of the US survey foot and recommended conversion to either the meter or the international foot.

As a rule of thumb in US water management, one acre-foot is taken to be the planned annual water usage of a suburban family household. In some areas of the desert Southwest, where water conservation is followed and often enforced, a typical family uses only about 0.25 acre-feet (310 m³) of water per year. One acre-foot per year is approximately 119.26 cubic feet per day (3.38 cubic meters per day).

The acre-foot per year has been used historically in the US in many water-management agreements, for example the Colorado River Compact, which divides 15 million acre-feet per year (19 cubic kilometers per year) among seven western US states.

Water reservoir capacities in the US are commonly given in thousands of acre-feet, abbreviated TAF or KAF.