An acre-foot is a unit of volume commonly used in the United States in reference to large-scale water resources, such as reservoirs, aqueducts, canals, sewer flow capacity, and river flows.
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It is defined by the volume of one acre of surface area to a depth of one foot. Since the acre is defined as a chain by a furlong (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 (exactly) | |
| = 325,851 3⁄7 U.S. gal (exactly) | |
| ≈ 271,328.072596 imp gal | |
| 1 U.S. survey acre foot | ≡ 43,560 U.S. survey cubic feet (by definition) |
| ≈ 1233.4892384681 m3 | |
| ≈ 325,853.383688 U.S. gal[nb 1] | |
| ≈ 271,329.700571 imp gal |
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.[1] 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.[2]
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, which divides 15 million acre-feet (MAF) per year or (586 m³/s) among seven western U.S. states.