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About these units
Acre (US Survey) (ac (US))
The US survey acre is based on the US survey foot, making it slightly different from the international acre. With a value of 43,560 survey-square feet, it appears extensively in historical property records and federal land surveys. Because millions of acres in the United States were mapped using PLSS, the survey acre remains necessary for interpreting legal land rights, even though modern mapping often uses metric units or the international foot. This specialized acre illustrates how subtle unit differences can survive for centuries due to legal and administrative inertia.
Acre (ac)
An acre is a traditional Anglo-American land unit equal to 43,560 square feet, or roughly 4,047 m². It originated from medieval English farming, where an acre represented the amount of land a yoke of oxen could plow in one day—reflecting its deep agricultural roots. The acre remains widely used in the United States and the UK (in certain contexts), especially in real estate, agriculture, and land conservation. It is culturally intuitive for rural populations, where land plots have been measured in acres for centuries. The unit's longevity demonstrates how historical agricultural practices shaped modern land evaluation systems. Despite its lack of coherence with the metric system, the acre endures because of its cultural familiarity and long-standing legal integration.