The acre is a unit of land area used in the imperial and US customary systems. It is traditionally defined as the area of one chain by one furlong (66 by 660 feet), which is exactly equal to 10 square chains, of a square mile, or 43,560 square feet, and approximately 4,047 m2, or about 40% of a hectare.
The international symbol of the acre is ac. In the United States both the international acre and the US survey acre are in use, but they differ by only two parts per million: see below. The most common use of the acre is to measure tracts of land. The acre, based upon the International yard and pound agreement of 1959, is defined as exactly 4,046.8564224 square metres.
One acre equals (0.0015625) square mile, 4,840 square yards, 43,560 square feet or about 4,047 square metres (0.4047 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. Originally, an acre was understood as a selion of land sized at forty perches (660 ft, or 1 furlong) long and four perches (66 ft) wide; this may have also been understood as an approximation of the amount of land a yoke of oxen could plough in one day. A square enclosing one acre is approximately 69.57 yards, or 208 feet 9 inches (63.61 metres) on a side. As a unit of measure, an acre has no prescribed shape; any area of 43,560 square feet is an acre.
Differences between international and US survey acres
In the international yard and pound agreement of 1959 the United States and five countries of the Commonwealth of Nations defined the international yard to be exactly 0.9144 metres. By inference, an "international acre" may be declared as exactly 4,046.8564224 square metres but it does not have a basis in any international agreement.
Both the international acre and the US survey acre contain of a square mile or 4,840 square yards, but alternative definitions of a yard are used (see survey foot and survey yard), so the exact size of an acre depends upon which yard it is based. The US survey acre is about 4,046.872609874252 square metres; its exact value ( 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.
Since the difference between the US survey acre and international acre is only about a quarter of the size of an A4 sheet of paper (0.016 square metres, 160 square centimetres or 24.8 square inches), it is usually not important which one is being discussed. Areas are seldom measured with sufficient accuracy for the different definitions to be detectable.
In India, residential plots are measured in square feet while agricultural land is measured in acres. In Sri Lanka, the division of an acre into 160 perches or 4 roods is common.
Its use as a primary unit for trade in the United Kingdom ceased to be permitted from 1 October 1995, due to the 1994 amendment of the Weights and Measures Act, where it was replaced by the hectare – though its use as a supplementary unit continues to be permitted indefinitely. This was with exemption of Land registration, which records the sale and possession of land, in 2010 HM Land Registry ended its exemption. The measure is still used to communicate with the public, and informally (non-contract) by the farming and property industries.
Equivalence to other units of area
1 international acre is equal to the following metric units:
0.40468564224 hectare (A square with 100 m sides has an area of 1 hectare.)
4,046.8564224 square metres
1 United States survey acre is equal to:
4,046.87261 square metres (1 square kilometre is equal to 247.105 acres)
1 acre (both variants) is equal to the following customary units:
A furlong by a chain (furlong 220 yards, chain 22 yards)
40 rods by 4 rods, 160 rods2 (historically fencing was often sold in 40 rod lengths)
(0.0015625) square mile (1 square mile is equal to 640 acres)
Perhaps the easiest way for US residents to envision an acre is as a rectangle measuring 88 yards by 55 yards ( of 880 yards by of 880 yards), about the size of a standard American football field.
The area of one acre (red) superposed on an American football field (green) and association football (soccer) pitch (blue).
To be more exact, one acre is 90.75% of a 100-yd-long by 53.33-yd-wide American football field (without the end zones). The full field, including the end zones, covers about 1.32 acres (0.53 ha).
It may also be remembered as 44,000 square feet, less 1%.
Farm-derived units of measurement:
The rod is a historical unit of length equal to yards. It may have originated from the typical length of a mediaeval ox-goad. There are 4 rods in one chain.
The furlong (meaning furrow length) was the distance a team of oxen could plough without resting. This was standardised to be exactly 40 rods or 10 chains.
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 and the value of river front access.
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.
A virgate was the amount of land tillable by two oxen in a ploughing season.
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 Englishæcer originally meaning "open field", cognate to west coast Norwegian ækre and Swedishåker, German Acker, Dutchakker, Latinager, Sanskritajr, and Greek (agros). In English, it was historically spelled aker.
It is ordained that 3 grains of barley dry and round do make an inch, 12 inches make 1 foot, 3 feet make 1 yard, 5 yards and a half make a perch, and 40 perches in length and 4 in breadth make an acre.
The acre was roughly the amount of land tillable by a yoke of oxen 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 there were many variants of "acre", differing between the German states:
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 acre is related to the square mile, with 640 acres making up one square mile. One mile is 5280 feet (1760 yards). In western Canada and the western United States, 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 mile (880 yards) and is square mile in area, or 160 acres. These subunits would typically then again be divided into quarters, with each side being mile long, and being 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 Canadian Prairie Provinces and the US Midwest are on square-mile grids for surveying purposes.
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 US 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.
^Minimum Standard Detail Requirements For ALTA/ACSM Land Title SurveysArchived 4 September 2013 at the Wayback Machine.. Federick, MD: American Congress on Surveying and Mapping. 2011. [The standard uses "precision" in a sense more typically used for "accuracy"; the stated maximum allowable "precision" (page 3) is 2 cm and 50 parts per million. An instrument consistently measuring 2 cm short would measure the area of a one international acre square, 63.614907 m on a side, as 4044.3 square metres, 2.6 square metres less than the true value, a far greater discrepancy than the difference between the international and survey acres.]