Acre

For the state in Brazil, see Acre (state).

An acre is the area of a rectangle, whose length is one furlong and width one chain.

Acre is an English unit of area, which is also used in the United States and some Commonwealth countries. It is most often used to describe areas of land.
 By the official definition it is 4,046.8564224 m2 (4,840 square yards). It is the area of one furlong by one chain. 
 The international acre is 4,046.8564224 m2, or 0.404686 hectares. This is based on international foot of 0.3048 m.
 The length of the side of a square, whose area is one acre, is 209.71 feet.

An acre is approximately equal:
 the area of an American football field.

Originally, it was seen as the area of land a farmer could plough with an ox in a day. Since this definition depended on the farmer and the ox, it looked variable. Therefore, Edward I, Edward III, Henry VIII and George IV passed laws fixing its size. The Commonwealth countries, including the United States then also introduced these laws.

Other websites
 The Units of Measurement Regulations 1995
 NIST Handbook 44 

Category:Units of area