The acre is a unit of area used in the imperial and U.S. customary systems. An acre is about 40% of a hectare – slightly smaller than an American football field. The acre is no longer commonly used, although a few notable exceptions include the United States, Australia, India, Burma and the United Kingdom (as of 2010, the acre is no longer officially used in the United Kingdom, though is still used in real estate descriptions). It is still used, to some extent, in Canada.
The international symbol of the acre is ac, and is defined as 1/640 of a square mile. The most commonly used acre today is the international acre. In the United States both the international acre and the slightly different US survey acre are in use. The most common use of the acre is to measure tracts of land. One international acre is defined as 4046.8564224 square metres.
One acre equals 0.0015625 square miles, 4,840 square yards, 43,560 square feet or about 4,047 square metres (0.405 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 on which yard it is based on. Originally, an acre was understood as a selion of land sized at forty perches (660 ft) 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 square enclosing one acre is approximately 69.57 yards, or 208 feet 9 inches (63.63 metres) on a side. As a unit of measure, an acre has no prescribed shape; any perimeter enclosing 43,560 square feet is an acre in size.
The acre is often used to express areas of land in the United States, Canada, and in countries where the Imperial System is still in use. As of 2010[update], the acre is not used officially in the United Kingdom but is still often seen on estate agents' boards. In the metric system, the hectare is commonly used for the same purpose. An acre is about 40% of a hectare.
Differences between international and U.S. survey acres 
In the international yard and pound agreement of 1959 the United States and five countries of the Commonwealth of Nations defined the length of the international yard to be exactly 0.9144 metres. Consequently, the international acre is exactly 4,046.8564224 square metres.
Both the international acre and the U.S. survey acre contain 1/640 of a square mile or 4,840 square yards, but there are alternative definitions of a yard (see survey foot and survey yard), so the exact size of an acre depends on which yard it is based on. The U.S. survey acre is about 4,046.872 609 874 252 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. Surveyors in the United States use both international and survey feet, and consequently, both varieties of acre.
Since the difference between the U.S. 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 stated to sufficient accuracy for the different definitions to be detectable.
South Asia 
In India, especially in South India, residential plots are measured in cents or decimel, which is one hundredth of an acre, or 435.60 square feet (40.469 m2). In Sri Lanka the division of an acre into 160 perches or 4 roods is common.
Equivalence to other units of area 
1 international acre is equal to the following metric units:
- 4,046.8564224 square metres
- 0.40468564224 hectare (A square with 100 m sides has an area of 1 hectare.)
1 United States survey acre is equal to:
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)
- 1⁄640 (0.0015625) square mile (1 square mile is equal to 640 acres)
Perhaps the easiest way for U.S residents to envisage 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 percent of a 100 yards (91.44 metres) long by 53.33 yards (48.76 metres) wide American football field (without the end zones). The full field, including the end zones, covers approximately 1.32 acres (0.53 ha).
It may also be remembered as 44,000 square feet, less 1%.
Historical origin 
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, and Greek αγρός (agros). In English it was historically spelt aker.
The acre was approximately 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, whereas in Germany as many variants of "acre" existed as there were German states.
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 and
- 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 1⁄2mile (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 1⁄16of 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 is on square mile grids for surveying purposes.
Other acres 
- 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 U.S. 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
- Roman acre = 1,260 square metres
- God's Acre – a synonym for a churchyard.
- The Byzantine zeugarion was also based on the area of farmland which could be ploughed by a pair (zeugos) of oxen; this was adopted as the Ottoman "çift" (also meaning "pair").
See also 
- Anthropic units
- Conversion of units
- Spanish customary units
- Quarter acre
- 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
- Section (United States land surveying)
- Public Land Survey System
- National Institute of Standards and Technology (n.d.) General Tables of Units of Measurement
- Klein, Herbert Arthur. The Science of Measurement: A Historical Survey. Dover Publications, 1988. Print. p. 76
- 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.
- "acre, n.2". OED Online. September 2011. Oxford University Press. (accessed November 15, 2012).
- Holland, Robert. (1886). A glossary of words used in the County of Chester. London: Trübner for the English Dialect Society. p. 3.
- The Collaborative International Dictionary of English.[dead link]
- Malcolm, Noel (1999). Kosovo: A Short History. Harper Perennial. ISBN 978-0-06-097775-7.
|Wikisource has the text of the 1911 Encyclopædia Britannica article Acre (land measure).|