An acre was originally defined as the amount of land a man and an ox could plow in a day!

The acre is a unit of area used in the imperial and U.S. Customary systems. An acre is about 40 % of a hectare – slightly smaller than an American football field. The acre is no longer commonly used in most countries.

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.

During the middle Ages, an acre was the amount of land that could be plowed in one day with a yoke of oxen and measure by one chain in width, and one furlong, or 10 chains in length, yielding 4840 square yards.