The acre is a unit of land area used in the British imperial and the United States customary systems. It is traditionally defined as the area of one chain by one furlong, which is exactly equal to 10 square chains, 1⁄640 of a square mile, 4,840 square yards, or 43,560 square feet, and approximately 4,047 m², or about 40% of a hectare. Based upon the international yard and pound agreement of 1959, an acre may be declared as exactly 4,046.8564224 square metres. The acre is sometimes abbreviated ac but is usually spelled out as the word "acre".