Acre

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.

One acre comprises 4,840 square yards or 43,560 square feet (which can be easily remembered as 44,000 square feet, less 1%; or as the product of 66 x 660). Because of alternative definitions of a yard or a foot, the exact size of an acre also varies slightly. Originally, an acre was understood as a selion of land sized at one furlong (660 ft) long and one chain (66 ft) wide; this may have also been understood as an approximation of the amount of land an ox could plow in one day. A square enclosing one acre is approximately 208 feet and 9 inches (63.6 meters) on a side. But as a unit of measure an acre has no prescribed configuration; any perimeter enclosing 43,560 ft2 is an acre in size.

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 53.33-yard-wide 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:
 * 4,046.8564224 square meters
 * 0.40468564224 hectare

1 United States survey acre is equal to:
 * 4,046.87261 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)
 * 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
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 αγϝος (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 metres, 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.