One square mile equals 640 survey acres. A survey acre is a U.S. customary unit of area derived from the Imperial system of measurement and is used primarily in the United States.
One acre contains 4,840 square yards and 43,560 square feet. The counterpart of the survey acre in the metric system is the hectare. One survey acre is the equivalent of slightly more than 0.4 hectares. An acre does not have any particular type of shape, and length and width can vary. The international acre is also 1/640 of a square mile, but the number of square meters can vary ever so slightly. For reference, an acre is about 4/5 the size of a football field with both end zones.