The acre (/ˈeɪkər/ AY-kər) is a unit of land area used in the British imperial and the United States customary systems. It is traditionally defined as the area of one chain by one furlong (66 by 660 feet), which is exactly equal to 10 square chains, 1⁄640 of a square mile, 4,840 square yards, or 43,560 square feet, and approximately 4,047 m, or about 40% of a hectare. The acre is sometimes abbreviated ac, but is usually spelled out as the word "acre".
Traditionally, in the Middle Ages, an acre was conceived of as the area of land that could be ploughed by one man using a team of eight oxen in one day. The acre is still a statutory measure in the United States, where both the international acre and the US survey acre are in use, but they differ by only four parts per million. The most common use of the acre is to measure tracts of land. The acre is used in many existing and former Commonwealth of Nations countries by custom. In a few, it continues as a statute measure, although not since 2010 in the UK, and not for decades in Australia, New Zealand, and South Africa. In many places where the acre is no longer a statute measure, it is still lawful to use as supplementary information next to the statutory hectare measurement.
