- For the state in Brazil, see Acre (state).
- By the official definition it is 4,840 square yards. It is the area of a rectangle whose long side is one furlong and short side 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 U.S. survey acre is 4,046.87261 m². This is based on the U.S. survey foot of 1200⁄3937 m.
An acre is equivalent to approximately:
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.