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Comparison of some Imperial and metric units of area
|Unit system||US customary units|
|1 ac in ...||... is equal to ...|
|SI units||4,046.9 m2|
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, 1⁄640 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. The acre is a statute measure in the United States and was formerly one in the United Kingdom and almost all countries of the former British Empire, although informal use continues.
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.
One acre equals 1⁄640 (0.0015625) square mile, 4,840 square yards, 43,560 square feet  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;  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.
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.  By inference, an "international acre" may be declared 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 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 4,046.872609874252 square metres; its exact value (4046 13,525,426/ m2) is based on an inch defined by 1 metre = 39.37 inches exactly, as established by the Mendenhall Order. Surveyors in the United States use both international and survey feet, and consequently, both varieties of acre. 
Since the difference between the US survey acre and international acre is only about a quarter of the size of an A4 sheet of paper (0.016 square metres, 160 square centimetres or 24.8 square inches), it is usually not important which one is being discussed. Areas are seldom measured with sufficient accuracy for the different definitions to be detectable. 
The acre is commonly used in a number of current and former Commonwealth countries by custom, and in a few it continues as a statutory measure for legal transactions. These include Antigua and Barbuda,  American Samoa,  The Bahamas,  Belize,  the British Virgin Islands,  the Cayman Islands,  Canada,  Dominica,  the Falkland Islands,  Grenada,  Ghana,  Guam,  the Northern Mariana Islands,  Jamaica,  Montserrat,  Samoa,  Saint Lucia,  St. Helena,  St. Kitts and Nevis,  St. Vincent and the Grenadines,  Turks and Caicos,  the United Kingdom, the United States and the US Virgin Islands. 
In India, residential plots are measured in square feet while agricultural land is measured in acres.[ citation needed] In Sri Lanka, the division of an acre into 160 perches or 4 roods is common.[ citation needed]
In Sindh (Pakistan), residential plots as well as open/agriculture land measurement is in Acres, Jareb, Wiswa and Gunta.[ citation needed]
Its use as a primary unit for trade in the United Kingdom ceased to be permitted from 1 October 1995, due to the 1994 amendment of the Weights and Measures Act,  where it was replaced by the hectare – though its use as a supplementary unit continues to be permitted indefinitely.  This was with exemption of Land registration,  which records the sale and possession of land,  in 2010 HM Land Registry ended its exemption.  The measure is still used to communicate with the public,  and informally (non-contract) by the farming and property industries.   
1 international acre is equal to the following metric units:
- 0.40468564224 hectare (A square with 100 m sides has an area of 1 hectare.)
- 4,046.8564224 square metres
1 United States survey acre is equal to:
- 0.404687261 hectare
- 4,046.87261 square metres (1 square kilometre is equal to 247.105 acres)
1 acre (both variants) is equal to the following customary units:
- 66 feet × 660 feet (43,560 square feet)
- 10 square chains (1 chain = 66 feet = 22 yards = 4 rods = 100 links)
- 1 acre is approximately 208.71 feet × 208.71 feet (a square)
- 4,840 square yards
- 43,560 square feet
- 160 perches. A perch is equal to a square rod (1 square rod is 0.00625 acre)
- 4 roods
- A furlong by a chain (furlong 220 yards, chain 22 yards)
- 40 rods by 4 rods, 160 rods2 (historically fencing was often sold in 40 rod lengths[ citation needed])
- 1⁄640 (0.0015625) square mile (1 square mile is equal to 640 acres)
Perhaps the easiest way for US residents to envision an acre is as a rectangle measuring 88 yards by 55 yards (1⁄10 of 880 yards by 1⁄16 of 880 yards), about 9⁄10 the size of a standard American football field. To be more exact, one acre is 90.75% of a 100-yd-long by 53.33-yd-wide American football field (without the end zone). The full field, including the end zones, covers about 1.32 acres (0.53 ha).
For residents of other countries, the acre might be envisioned as rather more than half of a 1.76 acres (0.71 ha) Association football (soccer) pitch.
It may also be remembered as 1% short of 44,000 square feet.
The word "acre" is derived from Old English æcer originally meaning "open field", cognate to west coast Norwegian ækre and Swedish åker, German Acker, Dutch akker, Latin ager, Sanskrit ajr, and Greek αγρός (agros). In English, it was historically spelled aker.
A possible wording of the Act on the Composition of Yards and Perches, dating from around 1300, is:
It is ordained that 3 grains of barley dry and round do make an inch, 12 inches make 1 foot, 3 feet make 1 yard, 5 yards and a half make a perch, and 40 perches in length and 4 in breadth make an acre.
The acre was roughly the amount of land tillable by a yoke of oxen in one day.  This explains one definition as the area of a rectangle with sides of length one chain and one furlong. A long, narrow strip of land is more efficient to plough than a square plot, since the plough does not have to be turned so often. The word "furlong" itself derives from the fact that it is one furrow long.
Before the enactment of the metric system, many countries in Europe used their own official acres. These were differently sized in different countries: for instance, the historical French acre was 4,221 square metres,[ citation needed] whereas in Germany there were many variants of "acre", differing between the German states:
|Place||Name||Area in m²
||Area in (local)|
|Prussia (1816–1869)||Magdeburg Morgen||2,553.22||180|
|Saxony (1781)||Morgen, Scheffel (Aussaat)||2,767||150|
|Grand Duchy of Baden (from 1810)||Badischer Morgen||3,600||400|
|Württemberg (1806–1871)||Schwäbischer Morgen||3,152||384|
|Bergisches Land||Bergischer Morgen||2,132||120|
|Cologne, Rhineland||Rheinländischer Morgen||3,176||150|
|Hanover (before 1836)||2,608||120|
|Hanover (from 1836)||2,621||120|
|Holstein||Tonne (Tønde)||5,046||240 QGeestR|
|Frankfurt am Main||Feldmorgen||2,025||160 QFeldR|
|Frankfurt am Main||Waldmorgen||3,256||160 QWaldR|
|Altes Land ( Harburg und Stade)||8,185|
Statutory values for the acre were enacted in England, and subsequently the United Kingdom, by acts of:
- Edward I
- Edward III
- Henry VIII
- George IV
- Queen Victoria – the British Weights and Measures Act of 1878 defined it as containing 4,840 square yards.
Historically, the size of farms and landed estates in the United Kingdom was usually expressed in acres (or acres, roods, and perches), even if the number of acres was so large that it might conveniently have been expressed in square miles. For example, a certain landowner might have been said to own 32,000 acres of land, not 50 square miles of land.
The acre is related to the square mile, with 640 acres making up one square mile. One mile is 5280 feet (1760 yards). In western Canada and the western United States, divisions of land area were typically based on the square mile, and fractions thereof. If the square mile is divided into quarters, each quarter has a side length of 1⁄2 mile (880 yards) and is 1⁄4 square mile in area, or 160 acres. These subunits would typically then again be divided into quarters, with each side being 1⁄4 mile long, and being 1⁄16 of a square mile in area, or 40 acres. In the United States, farmland was typically divided as such, and the phrase "the back 40" would refer to the 40-acre parcel to the back of the farm. Most of the Canadian Prairie Provinces and the US Midwest are on square-mile grids for surveying purposes.
- Customary acre – The customary acre was a measure of roughly similar size to the acre described above, but it was subject to considerable local variation similar to the variation found in carucates, virgates, bovates, nooks, and farundels. However, there were more ancient measures that were also farthingales. These may have been multiples of the customary acre, rather than the statute acre.
- Builder's acre – In US construction and real estate development, an area of 40,000 square feet. Used to simplify math and for marketing, it is nearly 10% smaller than a survey acre.
- Scottish acre, one of a number of obsolete Scottish units of measurement
- Irish acre = 7,840 square yards
- Cheshire acre = 10,240 square yards 
- Stremma or Greek acre ≈ 10,000 square Greek feet, but now set at exactly 1,000 square metres (a similar unit was the zeugarion) 
- Dunam or Turkish acre ≈ 1,600 square Turkish paces, but now set at exactly 1,000 square metres (a similar unit was the çift) 
- Actus quadratus or Roman acre ≈ 14,400 square Roman feet (about 1,260 square metres)
- God's Acre – a synonym for a churchyard. 
- Long acre – the grass strip on either side of a road that may be used for illicit grazing.
- Anthropic units
- Conversion of units
- French arpent – also used in Louisiana as length and area unit of measure
- a Morgen ("morning") of land is usually set at 2⁄3 of a Tagwerk ("day work") of ploughing with an ox
- Public Land Survey System
- Quarter acre
- Section (United States land surveying)
- Spanish customary units
- "Manuscripts and Special Collections - Measurements". the University of Nottingham. Retrieved 1 August 2018.
- National Institute of Standards and Technology (n.d.) General Tables of Units of Measurement Archived 26 November 2006 at the Wayback Machine
- Klein, Herbert Arthur (3 December 2012). The Science of Measurement: A Historical Survey. Courier Corporation. p. 76. ISBN 978-0-486-14497-9. Retrieved 29 June 2015.
- National Bureau of Standards. (1959). Refinement of Values for the Yard and the Pound.
- National Geodetic Survey, (January 1991), " Policy of the National Geodetic Survey Concerning Units of Measure for the State Plane Coordinate System of 1983.
- Minimum Standard Detail Requirements For ALTA/ACSM Land Title Surveys Archived 4 September 2013 at the Wayback Machine. Federick, MD: American Congress on Surveying and Mapping. 2011. [The standard uses "precision" in a sense more typically used for "accuracy"; the stated maximum allowable "precision" (page 3) is 2 cm and 50 parts per million. An instrument consistently measuring 2 cm short would measure the area of a one international acre square, 63.614907 m on a side, as 4044.3 square metres, 2.6 square metres less than the true value, a far greater discrepancy than the difference between the international and survey acres.]
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|Wikisource has the text of the 1911 Encyclopædia Britannica article Acre (land measure).|