The acre is a unit of area
in a number of different systems
Including the imperial and U.S. customary systems. The most commonly used acres today are the international acre and, in the
United States, the survey acre.
One international acre is equal to 4,046.8564224 m2. One U.S. survey acre is equal to 62,726,400,000⁄15,499,969 m2 = 4,046.8726098 m2.
The area of one acre (red) overlaid on an American football field One acre comprises 4,840 square yards or 43,560 square feet (which can be easily remembered as 44,000 square feet, less 1%). Because of alternative definitions of a yard or a foot, the exact size of an acre also varies slightly. Originally, an acre was a selion of land one furlong (660 ft) long and one chain (66 ft) wide; the measure appears to have begun as an approximation of the amount of land an ox could plow in one day. However, an acre is a measure of area, and has no particular width, length or shape.
The acre is often used to express areas of land. In the metric system, the hectare is commonly used for the same purpose. An acre is approximately 40% of a hectare.
One acre is 90.75 yards of a 53.33yardwide American football field. The full field, including the end zones, covers approximately 1.32 acres.
International acre
In 1958, the United States and countries of the Commonwealth of Nations defined the length of the international yard to be 0.9144 meters. Consequently, the international acre is exactly 4,046.8564224 square meters.
United States survey acre
The United States survey acre is approximately 4,046.873 square meters; its exact value (4046+13,525,426⁄15,499,969 m²) is based on an inch defined by 1 meter = 39.37 inches exactly, as established by the Mendenhall Order.
Equivalence to other units of area
1 international acre is equal to the following metric units:
 4046.8564224 square meters
 0.40468564224 hectare
1 United States survey acre is equal to:
 4046.87261 square meters
 0.404687261 hectare
1 acre (both variants) is equal to the following customary units:
 66 feet × 660 feet (43,560 square feet)
 1 chain × 10 chains (1 chain = 66 feet = 22 yards = 4 rods = 100 links)
 100 links × 1,000 links = 100,000 square links
 1 acre is approximately 208.71 feet × 208.71 feet (a square)
 4,840 square yards
 160 perches. A perch is equal to a square rod (1 square rod is 0.00625 acre)
 10 square chains
 4 roods
 A chain by a furlong (chain 22 yards, furlong 220 yards)
 0.0015625 square mile (1 square mile is equal to 640 acres)
1 international acre is equal to the following Indian unit:
 100 Indian cents (1 cent is equal to 0.01 acre)
Historical origin
Farm derived units of measurement:
The rod is a historical unit of length equal to 5.5 yards. It may have originated from the typical length of a mediaeval oxgoad.
The furlong (meaning furrow length) was the distance a team of oxen could plow without resting. This was standardized to be exactly 40 rods.
An acre was the amount of land tillable by one man behind one ox in one day. Traditional acres were long and narrow due to the difficulty in turning the plow.
An oxgang was the amount of land tillable by one ox in a plowing season. This could vary from village to village, but was typically around 15 acres.
A virgate was the amount of land tillable by two oxen in a plowing season.
A carucate was the amount of land tillable by a team of eight oxen in a plowing season. This was equal to 8 oxgangs or 4 virgates.
The word "acre" is derived from Old English æcer (originally meaning "open field", cognate to west coast Norwegian language "ækre" and Swedish "åker", German Acker, Latin ager and Greek αγρος (agros).
The acre was approximately the amount of land tillable by one man behind an ox 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
meters, whereas in Germany as many variants of "acre" existed as there were German states.
Statutory values for the acre were enacted in England by acts of:
 Edward I,
 Edward III,
 Henry VIII,
 George IV and
 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.
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 farundells. However, there were more ancient measures that were also farthingales. These may have been multiples of the customary acre, rather than the statute acre.
Other acres
 Scottish acre, one of a number of obsolete Scottish units of measurement
 Irish acre
 Roman acre = 1,260 square meters
 God's Acre  a synonym for a churchyard.
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