An acre-foot is a unit of volume

Volume

Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....

 commonly used in the United States in reference to large-scale water resources, such as reservoirs, aqueduct

Aqueduct

An aqueduct is a water supply or navigable channel constructed to convey water. In modern engineering, the term is used for any system of pipes, ditches, canals, tunnels, and other structures used for this purpose....

s, canal

Canal

Canals are man-made channels for water. There are two types of canal:#Waterways: navigable transportation canals used for carrying ships and boats shipping goods and conveying people, further subdivided into two kinds:...

s, sewer

Sanitary sewer

A sanitary sewer is a separate underground carriage system specifically for transporting sewage from houses and commercial buildings to treatment or disposal. Sanitary sewers serving industrial areas also carry industrial wastewater...

 flow capacity, and river flows.

Definitions

It is defined by the volume of one acre

Acre

The acre is a unit of area in a number of different systems, including the imperial and U.S. customary systems. The most commonly used acres today are the international acre and, in the United States, the survey acre. The most common use of the acre is to measure tracts of land.The acre is related...

 of surface area to a depth of one foot. Since the acre is defined as a chain

Chain (unit)

A chain is a unit of length; it measures 66 feet or 22 yards or 100 links . There are 10 chains in a furlong, and 80 chains in one statute mile. An acre is the area of 10 square chains...

 by a furlong

Furlong

A furlong is a measure of distance in imperial units and U.S. customary units equal to one-eighth of a mile, equivalent to 220 yards, 660 feet, 40 rods, or 10 chains. The exact value of the furlong varies slightly among English-speaking countries....

 (66 ft × 660 ft) the acre-foot is exactly 43,560 cubic feet. Thus there are two definitions of the acre foot (differing by about 0.0006%) depending on whether we use the international or U.S. survey foot.

1 international acre foot	≡ 43,560 international cubic feet (by definition)
|= 1233.48183754752 m3 Cubic metre The cubic metre is the SI derived unit of volume. It is the volume of a cube with edges one metre in length. An alternative name, which allowed a different usage with metric prefixes, was the stère...  (exactly)
	= U.S. gal (exactly)
|≈ 1 acre.foot
1 U.S. survey acre foot	≡ 43,560 U.S. survey cubic feet (by definition)
|≈ m3
|≈ m3This conversion assumes that we use the international foot to define the U.S. gallon and the U.S. survey foot to define the acre foot. If we use the same foot for both, we get the U.S. gal figure as obtained previously.
|≈ m3

Discussion

As a rule of thumb in U.S. water management, one acre-foot is taken to be the planned water usage of a suburban family household, annually. In the desert South West, where water conservation is followed and often enforced, a typical family uses only about 0.25 acre-feet of water per year.

The acre-foot (or more specifically the time rate unit of acre-foot per year) has been used historically in the U.S. in many water-management agreements, for example the Colorado River Compact

Colorado River Compact

The Colorado River Compact is a 1922 agreement among seven U.S. states in the basin of the Colorado River in the American Southwest governing the allocation of the water rights to the river's water among the parties of the interstate compact...

, which divides 15 million acre-feet (MAF) per year or (586 m³/s) among seven western U.S. states.