المعهد الوطني للمعايير والتكنولوجيا
The most widely used area unit with a name unrelated to any length unit is the acre. The National Institute of Standards and Technology contends that customary area units are defined in terms of the square survey foot, not the square international foot. Conversion factors are based on Astin (July 27, 1968) and National Institute of Standards and Technology (2008).