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The acre is a unit of area in a number of different systems, including the imperial and US customary systems. The most commonly used acres today are the international acre and, in the United States, the survey acre.

One acre comprises 4,840 square yards or 43,560 square feet. 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 long and one chain wide. 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 53.33-yard-wide American football field. The full field, including the end zones, covers approximately 1.32 acres.

Contents 1 International acre 2 United States survey acre 3 Equivalence to other units of area 4 Historical origin 5 Customary acre 6 Other acres 7 References 8 See also 9 External links

In 1958, the United States and countries of the Commonwealth of Nations defined the length of the international yard to be 0.9144 meters.^[1] Consequently, the international acre is exactly 4046.8564224 square meters.

The United States survey acre is approximately 4046.873 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. It is the standard acre in the United States, but the fractional difference from the international acre is only 4 millionths, or 4 ten-thousandths of one percent.

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)
• 4840 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)

The word "acre" is derived from Old English æcer (originally meaning "open field", cognate to German Acker, Latin ager and Greek αγρος (agros).

The acre was selected as 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.

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.

The customary acre was a measure of roughly similar size to the acre described above, but was subject to considerable local variation. However, there were more ancient measures that were also used, including carucates, virgates, bovates, nooks, and farundells or farthingales. These may have been multiples of the customary acre, rather than the statute acre.

• Scottish acre, one of a number of obsolete Scottish units of measurement

• Conversion of units
• Acre-foot
• Obsolete Spanish and Portuguese units of measurement

• The Units of Measurement Regulations 1995 (United Kingdom)
• Cockeyed.com presents "How much is inside an acre?"