|Unit system||US Customary units|
|1 ac⋅ft in ...||... is equal to ...|
|SI units||≈ 1,233.5 m3|
|US customary units||43,560 cu ft|
|US customary units||≈ 325,850 US gal|
|Imperial units||≈ 271,330 imp gal|
The acre-foot is a non-SI unit of volume equal to about 1,233 m3 commonly used in the 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 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 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 are two definitions of an acre-foot (differing by about 0.0006%), depending on whether the "foot" used is an "international foot" or a "U.S. survey foot".
|1 acre-foot||= 43,560 cubic feet = 75,271,680 cu in|
|1 international acre-foot||= 43,560 international cubic feet|
|= 1,233.48183754752 m3|
|≈ 271,328.072596 imp gal|
|= 325,851+3⁄7 US gal|
|1 U.S. survey acre-foot||= 43,560 U.S. survey cubic feet|
|≈ 1233.4892384681 m3|
|≈ 271,329.700571 imp gal|
|≈ 325,853.383688 U.S. gal[a]|
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.[b] In some areas of the desert Southwest, where water conservation is followed and often enforced, a typical family uses only about 0.25 acre-foot of water per year. One acre-foot/year is approximately 119 cu ft/d (3.38 m3/d).
The acre-foot (or more specifically the time rate unit of 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 (590 cubic metres per second) among seven western US states.
Water reservoir capacities in the US are commonly given in thousands of acre-feet, abbreviated TAF or KAF.
In most other countries except the US, the metric system is in common use and water volumes are normally expressed in litre, cubic metre or cubic kilometre. One acre-foot is approximately equivalent to 1.233 megalitres. Large bodies of water may be measured in cubic kilometres (1,000,000,000 m3, or 1000 gigaliter), with 1 million acre-feet approximately equalling 1.233 km3.