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.
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.