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

An acre-foot volume (not drawn to scale)
General information
Unit systemUS Customary units
Unit ofVolume
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

An acre-foot equals approximately an 8 lane swimming pool, 25 meter long, 16 m wide and 3 m deep.

Government policy in the United States, as stated by law, is to phase out imperial and US customary unit and replace them with metric equivalents.[2] As of 2018, the metric system is mandatory to use in trade and commerce in all countries except the United States, Myanmar and Liberia.[3]


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 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 37 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[nb 1]


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.[4] 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.[5] One acre-foot/year is approximately 893 gallons (3.38 m3) per day.

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 (586 m3/s) among seven western U.S. states.

Water reservoir capacities in the U.S. are commonly given in thousands of acre-feet, abbreviated TAF.

In most other countries except USA, 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. As of 2018, the metric system is mandatory to use in trade and commerce in all countries except USA, Myanmar and Liberia.[6]

See also


  1. This conversion assumes the international foot is used to define the U.S. gallon and the U.S. survey foot to define the acre foot. If the same foot is used for both, the result is the 325,851 37 U.S. gal figure as obtained previously.


  1. "NM OSE Glossary". Archived from the original on 14 November 2005. Retrieved 15 March 2014.
  2. "US 1988 law on metrification". Retrieved 2019-09-21.
  3. "Countries not using SI". Retrieved 2019-09-21.
  4. The state of Montana assumes 1.0 acre-foot per year for a family of five. See Water Rights Bureau; state of Montana (April 13, 2004). "Form No. 627 R8/03 Notice of Water Right" (PDF). Archived from the original (PDF) on 2008-01-25. Retrieved 2008-01-30.
  5. Santa Fe, New Mexico rate averages 0.25 acre-foot per year per household. See Planning Division, Planning & Land Use Department, City of Santa Fe, New Mexico (February 2001). "Water Use in Santa Fe: A survey of residential and commercial water use in the Santa Fe urban area" (PDF). Retrieved 2008-01-30.
  6. "Countries not using SI". Retrieved 2019-09-21.

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