Science Fair Project Encyclopedia
- This article is about the unit of measure known as the acre. For other definitions, see Acre (disambiguation).
An acre is a measure of land area in Imperial units or U.S. customary units. It is equal to 43 560 square feet, or 4840 square yards. The precise meaning of this depends on the exact definition adopted for a foot: the international acre is 4 046.856 433 4 m² (for the UK, see ). For measurements based specifically on the US survey foot the US survey acre is ca. 4 046.872 610 m².
An area of land one furlong (220 yards) long and one chain (22 yards) wide or a square with about 208¾ feet on each side has an area of one acre. A square mile has 640 acres, and a square parcel of land ¼ mile wide has 40 acres. A square parcel of land ½ mile on a side (¼ square mile) is 160 acres, the usual land tract under the Homestead Act in the United States. This results in common field lengths of ½ mile, with every rod in width equal to one acre.
In metric units, an acre is approximately 0.4047 hectare; to a good enough approximation for many purposes, it can be taken as 0.40 hectare or 4.0 km²/1000 acres. Conversely, a hectare is about 2.5 acres (2.471 acres).
The acre was selected as approximately the amount of land tillable by one man behind an ox in one day. This explains its definition in terms of the non-square one-chain by one-furlong parcel of land; a long narrow strip of land is more efficient to plough than a square plot, since the plough does not have to be turned so often. Statutory values were enacted in England by acts of
- Edward I,
- Edward III,
- Henry VIII,
- George IV;
- Victoria - the British "Weights and Measures Act" of 1878 defined it as containing 4840 square yards.
An acre-foot is a measure of volume used to measure large volumes of water, an acre-foot being the volume necessary to cover one acre to a depth of one foot, 43,560 cubic feet (1233.5 cubic meters or 325 850 U.S. gallons). The volume of a reservoir may be measured in acre feet, for example. The acre-inch (102.79 m³, 27 154 U.S. gal) is also used.
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