Acre - Wikipedia

In the international yard and pound agreement of 1959, the United States and five countries of the Commonwealth of Nations defined the international yard to be exactly 0.9144 metre.[5] The US authorities decided that, while the refined definition would apply nationally in all other respects, the US survey foot (and thus the survey acre) would continue 'until such a time as it becomes desirable and expedient to readjust [it]'.[5] By inference, an "international acre" may be calculated as exactly 4,046.8564224 square metres but it does not have a basis in any international agreement.

Both the international acre and the US survey acre contain 1⁄640 of a square mile or 4,840 square yards, but alternative definitions of a yard are used (see Foot (unit) § Survey foot and Yard § Conversions), so the exact size of an acre depends upon the yard upon which it is based. The US survey acre is about 4,046.872 square metres; its exact value (⁠4046+13,525,426/15,499,969⁠ m2) is based on an inch defined by 1 metre = 39.37 inches exactly, as established by the Mendenhall Order of 1893.[6] Surveyors in the United States use both international and survey feet, and consequently, both varieties of acre.[7]

Since the difference between the US survey acre and international acre (0.016 square metres, 160 square centimetres or 24.8 square inches), is only about a quarter of the size of an A4 sheet or US letter, it is usually not important which one is being discussed. Areas are seldom measured with sufficient accuracy for the different definitions to be detectable.[8] In October 2019, the US National Geodetic Survey and the National Institute of Standards and Technology announced their joint intent to end the "temporary" continuance of the US survey foot, mile, and acre units (as permitted by their 1959 decision, above), with effect from the end of 2022.[9][10]

Spanish acre

The Puerto Rican cuerda (0.39 ha; 0.97 acres) is sometimes called the "Spanish acre" in the continental United States.[11]