The acre is a unit of area in a number of different systems, including the imperial and U.S. customary systems. Its international symbol is ac.

The most commonly used acres today are the international acre and, in the United States, the survey acre. The most common use of the acre is to measure tracts of land. One international acre is equal to 4046.8564224 square metres.

During the middle ages, an acre was the amount of land that could be plowed in one day with an ox.^[1]

Description[link]

The acre is related to the square mile, with 640 acres making up one square mile. One mile is 5280 feet (1760 yards). Divisions of land area were typically based on the square mile, and fractions thereof. If the square mile is divided into quarters, each quarter has a side length of ¹⁄₂ mile (880 yards) and is ¹⁄₄ square mile in area, or 160 acres. These subunits would typically then again be divided into quarters, with each side being ¹⁄₄ mile long, and being ¹⁄₁₆ of a square mile in area, or 40 acres. In the United States, farmland was typically divided as such, and the phrase "the back 40" would refer to the 40 acre parcel to the back of the farm. Most of the US midwest is on square mile grids for surveying purposes.

One acre equals 4,840 square yards, 43,560 square feet^[2] or about 4,047 square metres (0.405 hectares) (see below). While all modern variants of the acre contain 4,840 square yards, there are alternative definitions of a yard, so the exact size of an acre depends on which yard it is based on. 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 plough in one day. A square enclosing one acre is approximately 69.57 yards, or 208 feet 9 inches (63.63 metres) on a side. As a unit of measure, an acre has no prescribed shape; any perimeter enclosing 43,560 square feet is an acre in size.

The acre is often used to express areas of land in the United States and in countries where the Imperial System is still in use. As of 2010^[update], the acre is not used officially in the United Kingdom. In the metric system, the hectare is commonly used for the same purpose. An acre is approximately 40% of a hectare.

Perhaps the easiest way to envisage an acre is as a rectangle measuring 88 yards by 55 yards (¹⁄₁₀ of 880 yards by ¹⁄₁₆ of 880 yards), approximately the size of a standard American football field. To be more exact, one acre is 90.75 percent of a 100 yards (91.44 metres) long by 53.33 yards (48.76 metres) wide American football field (without the end zones). The full field, including the end zones, covers approximately 1.32 acres (0.53 ha). The acre is also approximately 56.68 percent of a 105 metres (344.49 feet) long by 68 metres (223.10 feet) wide Association football (soccer) pitch. It may also be remembered as 44,000 square feet, less 1%; or as the product of 66 × 660.

International acre[link]

In 1958, the United States and countries of the Commonwealth of Nations defined the length of the international yard to be 0.9144 metres.^[3] Consequently, the international acre is exactly 4,046.8564224 square metres. Since the difference between the U.S. and International acre is only approximately 0.016 square metres (24.8 square inches), it is usually not important which one is being discussed.

U.S. survey acre[link]

The U.S. survey acre is approximately 4,046.872 609 874 252 square metres; its exact value (4046 ^13,525,426⁄_15,499,969 m²) is based on an inch defined by 1 metre = 39.37 inches exactly, as established by the Mendenhall Order. Surveyors in the United States use both international and survey feet, and consequently, both varieties of acre.^[4] Areas are seldom stated to sufficient accuracy for the different definitions to be detectable.

South Asia[link]

In India, especially in South India, residential plots are measured in cents or decimel, which is one hundredth of an acre, or 435.60 square feet (40.469 m²). In Sri Lanka the division of an acre into 160 perches or 4 roods is common.^{[citation needed]}

Equivalence to other units of area[link]

1 international acre is equal to the following metric units:

• 4,046.8564224 square metres
• 0.40468564224 hectare (A square with 100 m sides has an area of 1 hectare.)

1 United States survey acre is equal to:

• 4,046.87261 square metres
• 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)
• ¹⁄₆₄₀ (0.0015625) square mile (1 square mile is equal to 640 acres)

Historical origin[link]

Farm-derived units of measurement:

1. The rod is a historical unit of length equal to 5½ yards. It may have originated from the typical length of a mediaeval ox-goad.
2. The furlong (meaning furrow length) was the distance a team of oxen could plough without resting. This was standardised to be exactly 40 rods.
3. 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 plough.
4. An oxgang was the amount of land tillable by one ox in a ploughing season. This could vary from village to village, but was typically around 15 acres.
5. A virgate was the amount of land tillable by two oxen in a ploughing season.
6. A carucate was the amount of land tillable by a team of eight oxen in a ploughing 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 ækre and Swedish åker, German Acker, Dutch akker, Latin ager, and Greek αγρός (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, and, subsequently, the United Kingdom, by acts of:

• Edward I,
• Edward III,
• Henry VIII,
• George IV and
• Queen 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.

Other acres[link]

• 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 farundels. However, there were more ancient measures that were also farthingales. These may have been multiples of the customary acre, rather than the statute acre.
• Builder's acre - In U.S. construction and real estate development, an area of 40,000 square feet. Used to simplify math and for marketing, it is nearly 10% smaller than a survey acre.
• Scottish acre, one of a number of obsolete Scottish units of measurement
• Irish acre = 7,840 square yards
• Cheshire acre = 10,240 square yards^[5]
• Roman acre = 1,260 square metres
• God's Acre – a synonym for a churchyard.^[6]

See also[link]

• Anthropic units
• Conversion of units
• Acre-foot
• Spanish customary units
• Quarter acre
• French arpent—also used in Louisiana as length and area unit of measure
• a Morgen ("morning") of land is usually set at ²⁄₃ of a Tagwerk ("day work") of ploughing with an ox

References[link]

External links[link]

Wikisource has the text of the 1911 Encyclopædia Britannica article Acre (land measure).

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