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 4046.8564224 m². One U.S. survey acre is equal to frac: 15,499,969 m² = 4046.8726098 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%). 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. 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.

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 4046.8564224 square meters.

United States survey acre

The United States survey acre is approximately 4046.873 square meters; its exact value ( 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 40 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 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 x 10 chains ( 1-chain = 66 feet or 22 yards or 4 rods)
• 1 acre is approximately 208.71 feet x 208.71 feet (square)
• 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)

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 αγρος (agros).