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acre
Comparison land area units.svg
Comparison of some Imperial and metric units of area
General information
Unit systemUS customary units, Imperial units
Unit ofArea
Symbolacre or ac 
Conversions
1 acre in ...... is equal to ...
   SI units   ≈ 4,046.9 m2
   US customary, Imperial   ≡ 4,840 sq yd
≡ ​1640 sq mi

The acre is a unit of land area used in the imperial and US customary systems. It is traditionally defined as the area of one chain by one furlong (66 by 660 feet), which is exactly equal to 10 square chains, ​1640 of a square mile, 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. One recognised symbol for the acre is ac,[1] but the word "acre" is also used as the symbol.[2]

The acre is a statute measure in the United States, in the United Kingdom and in almost all countries of the former British Empire: in most it is lawful to "use for trade" if given as supplementary information and is not used for land registration.

In the United States both the international acre and the US survey acre are in use, but they differ by only two parts per million, see below. The most common use of the acre is to measure tracts of land.

Traditionally, in the Middle Ages, an acre was defined as the area of land that could be ploughed in one day by a yoke of oxen.[3]

Description

One acre equals ​1640 (0.0015625) square mile, 4,840 square yards, 43,560 square feet,[2] or about 4,047 square metres (0.4047 hectares) (see below). While all modern variants of the acre contain 4,840 square yards, there are alternative definitions of a yard, so the exact size of an acre depends upon the particular yard on which it is based. Originally, an acre was understood as a selion of land sized at forty perches (660 ft, or 1 furlong) long and four perches (66 ft) wide;[4] this may have also been understood as an approximation of the amount of land a yoke of oxen could plough in one day (a furlong being "a furrow long"). A square enclosing one acre is approximately 69.57 yards, or 208 feet 9 inches (63.61 metres), on a side. As a unit of measure, an acre has no prescribed shape; any area of 43,560 square feet is an acre.

US survey acres

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 ​1640 of a square mile or 4,840 square yards, but alternative definitions of a yard are used (see survey foot and survey yard), so the exact size of an acre depends upon which yard 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 1922. Surveyors in the United States use both international and survey feet, and consequently, both varieties of acre.[6]

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.[7]

In October 2019, U.S. National Geodetic Survey and 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.[8][9]

Spanish acre

The acre is a unit of land area used in the imperial and US customary systems. It is traditionally defined as the area of one chain by one furlong (66 by 660 feet), which is exactly equal to 10 square chains, ​1640 of a square mile, 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. One recognised symbol for the acre is ac,[1] but the word "acre" is also used as the symbol.[2]

The acre is a statute measure in the United States, in the United Kingdom and in almost all countries of the former British Empire: in most it is lawful to "use for trade" if given as supplementary information and is not used for land registration.

In the United States both the international acre and the US survey acre are in use, but they differ by only two parts per million, see below. The most common use of the acre is to measure tracts of land.

Traditionally, in the Middle Ages, an acre was defined as the area of land that could be ploughed in one day by a yoke of oxen.[3]