An acre is a unit of area primarily used in the imperial and U.S. customary measurement systems. It is equal to 4,047 square meters, 4,840 square yards, 0.405 hectares, 1/640 or 0.0015625 square miles. The acre is defined in square units because it is a measure of area and not length, and it can take any shape. A square measurement consists of a two-dimensional derivative of a linear measure, so a square foot refers to the area of a square with one-foot length sides.