A unit of area in the English system of measure, defined as 10 square chains (1 chain is 4 rods or 66 feet). It is exactly equal to 43,560 square feet or 4,840 square yards and is approximately equal to 4046. 8564224 square meters. There are 640 acres in one square mile. Although acre may be defined otherwise in some localities, these are local definitions and the word is usually qualified as builder's acre or Block Island acre. The English system of measures contains no legally defined unit of area. The size of the acre therefore depends on the length of the English yard, which is legally defined. By an ordinance of Edward I in 1303, the acre was defined as the area contained in a rectangle 40 rods long and 4 rods wide. With the rod defined as 5. 5 ulnae (yards) as defined by Edward I's iron standard for the ulna, the acre is 4840 square yards.