Some facts about Acre
The acre is a unit of land area used in the imperial and US customary systems. It is defined as the area of 1 chain by 1 furlong (66 by 660 feet), which is exactly equal to 1⁄640 of a square mile, 43,560 square feet, approximately 4,047 m2, or about 40% of a hectare.
The acre is commonly used in Antigua and Barbuda, Australia, American Samoa, the Bahamas, Belize, the British Virgin Islands, the Cayman Islands, Canada, Dominica, the Falkland Islands, Grenada, Ghana, Guam, the Northern Mariana Islands, India, Sri Lanka,Bangladesh,Nepal, Ireland, Jamaica, Montserrat, Myanmar, Pakistan, Samoa, St. Lucia, St. Helena, St. Kitts and Nevis, St. Vincent and the Grenadines, Turks and Caicos, the United States and the US Virgin Islands.
The international symbol of the acre is ac. The most commonly used acre today is the international acre. In the United States both the international acre and the US survey acre are in use, but differ by only two parts per million; see below. The most common use of the acre is to measure tracts of land. One international acre is defined as exactly 4,046.8564224 square metres.
An acre was defined in the Middle Ages as the amount of land that could be ploughed in one day by a yoke of oxen.
More info on Wikipedia