An acre is the name of a unit of area in a number of different systems, including Imperial units, and United States customary units. The most commonly used acres today are the international acre and, in the United States, the survey acre. There are 43,560 square feet in an acre. There are always 4,840 square yards in an acre. Because of alternative definitions of a yard, the exact size of an acre also varies. Related units of length are the acre's length of one furlong (220 yards) and the acre's breadth of one chain (4 rods or 22 yards).