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Square decametre (International spelling as used by the International Bureau of Weights and Measures) or square decameter (American spelling) or are, symbol dam2, is the unit of area or surface area.
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.