Acre

This entry is about the unit of area. For other meanings see Acre (disambiguation)

An acre is the name of a unit of area in a number of different systems, including Imperial units and United States customary units. 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. Related units of length are the acre's length of one furlong (220 yards) and the acre's breadth of one chain (4 rods or 66 feet).

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 0.4 hectares.

One acre is slightly less than 91 yards on an American football field, with the full field, including the end zones, covering 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 metres.^[1] Consequently, the international acre is exactly 4046.8564224 square metres.

United States survey acre

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.

Equivalence to other units of area

1 international acre is equal to the following metric units:

• 4046.8564224 square metres
• 0.40468564224 hectares

1 United States survey acre is equal to:

• 4046.87261 square metres
• 0.404687261 hectares

1 acre (both variants) is equal to the following customary units:

• 4840 square yards
• 160 perches. A Perch is equal to a square rod (1 square rod is 0.00625 acres)
• 10 square chains
• 4 rood
• A chain by a furlong (chain 22 yards, furlong 220 yards)
• 0.0015625 square miles (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 acres)

Use of the acre

In the United Kingdom the use of acres is now officially discouraged, but it remains a familiar measure of land with the general public and the still standard description of land in business. The acre also remains the legal unit of land measure in the United States.

The usual land tract under the Homestead Act in the United States is 160 acres or 0.25 square miles. This results in common field lengths of 0.5 miles, with every rod in width equal to one acre.

The area of land is usually determined by reference to the area within its boundaries as drawn on a map. On level ground, the area of the terrain will correspond to the area on the map. On sloping ground, the area of the terrain will be greater than the area on the map.

Historical origin

The word "acre" is derived from Old English æcer (originally meaning "open field", cognate to German Acker, Latin ager and Old Greek 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 always expressed in acres, 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.

Other acres

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

References

1. ^ National Bureau of Standards. Refinement of Values for the Yard and the Pound.

See also

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

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