An acre is the amount of land that can be plowed by a pair of oxen in one day.
The acre is a unit of area used in the imperial and U.S. customary systems. It is equivalent to 43,560 square feet (approximately 4,047 m2). An acre is about 40% of a hectare or about 75% the size of an American football field.
The acre is commonly used in the United States, Antigua and Barbuda American Samoa, The Bahamas, Belize, the British Virgin Islands, the Cayman Islands, Dominica, the Falkland Islands, Grenada, Guam, the Northern Mariana Islands, India, Jamaica, Montserrat, Myanmar, Pakistan, Samoa, St. Lucia, St. Helena, St. Kitts and Nevis, St. Vincent and the Grenadines, Turks and Caicos, and the US Virgin Islands.
The international symbol of the acre is ac, and is defined as 1/640 of a square mile. The most commonly used acre today is the international acre. In the United States both the international acre and the slightly different US survey acre are in use. The most common use of the acre is to measure tracts of land. One international acre is defined as exactly 4046.8564224 square metres.
During the Middle Ages, an acre was the amount of land that could be ploughed in one day with a yoke of oxen and measured by one chain in width (22 yards), and one furlong, or 10 chains in length (220 yards), yielding 4840 square yards.