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Physical unit
Unit name acre
Unit symbol
Physical quantity (s) Area
Formula symbol
system Anglo-American system of measurement
In SI units
Named after Old English æcer "Acker, Feld"
Derived from Rod , Furlong , Foot

Acre ( plural German Acre or Acres; English acre , plural acres ) is one of the British Isles derived Anglo-American unit for area determination of land and is roughly 4047  or 40.47 Ar . In addition to the acre, in today's Anglo-American area measurement system, practically only the square foot unit is used for land and ground surveying ; in the United States , the size of a property is measured in these two units of area alone. These units are also widely used in Great Britain , Canada , India , Australia and other Commonwealth countries, although there is now a main metric system of measurements.



Comparison of some units of area with the acre.

Nowadays the acre is directly defined as 43,560 square feet because the length measures furlong (= 660 feet) and chain (= 66 feet) used for the original definition are now uncommon.

1 acre (international) = 43,560 square feet = 4046.8564224 m² ≈ 4046.9 m²
1 acre (US survey) = 43,560 square survey feet 4046.8726099 m² ≈ 4046.9 m² (until December 31, 2022)

Taking into account the mile and the nowadays no longer used area dimensions Rood and Square Rod (also called Square Perch , Perch, Square Pole, Pole), the following conversion results:

1/640 square mile = 1 acre = 4 roods = 160 square rods = 43,560 square feet

Practical use

The acre is the main measure of land area. As a rule, no more than two decimal places are given, giving an accuracy of ± 20 m². As soon as more precise information is required, for example in the case of building land, the square foot unit is used.

In the case of land used for agriculture, the areas are divided into workable acre and non-workable acre . This means the actual usable area and the area not usable for agricultural purposes, such as wasteland .

Very large areas are also given in acre, for example 87,000 ac (≈ 350 km²). There is usually no conversion to square miles.

Acre as the basis for derived units

  • In agriculture, the area yield refers to the acre. In the case of grain, the yield is given in bushels / acre . (See main article Bushel (grain measure) )
  • Land of all types is priced in $ / acre in the United States and Canada .
Conversion from $ / acre to $ / ha :  $ 1 / acre = $ 2.47105 / ha 
Conversion from $ / ha to $ / acre :  $ 1 / ha = $ 0.40469 / acre 
  • The acre-foot and the acre-inch are units of volume for large amounts of water in the United States, for example in municipal water consumption or agricultural irrigation.


Original definition

The unit acre, from Old English æcer , field, originally referred to the strip of land that could be plowed in one day by a team of oxen. Under King Edward I and again under Edward III. and Henry VIII , the acre was legally established as a piece of land 40 rods (or perches; = 1  furlong or 660 feet) and 4 rods (or perches; = 66 feet or [since 1620] 1 chain ) wide  ) or 160 square rods, whatever the shape of the property is defined.

With its size of roughly 40  ares , the acre is typologically comparable with the morning , the daily work (or Tagwan ), the yoke (or Juchart ) and the man work .

US survey foot 1959-2022

The Anglo-American units of length, which differ throughout the world, were standardized in 1959 . In the United States, this resulted in an increase in the length measures used up to then by a factor of 1.000002 compared to the new "international" length measures. In the case of land surveying, this would have led to noticeable differences at great distances. For a route length of 1000 km the difference would be 2 m. In order not to have to change the existing values, the old system of measurement was therefore retained in the United States - only for the purpose of land surveying - and the previously used measure of length Foot was given the name Survey Foot .

The US Survey Foot will be abolished on December 31, 2022. From then on, only the International Foot may be used, which is officially referred to as the Foot from this point in time .

In the United States, the acre unit was based on the Survey Foot until December 31, 2022, otherwise the international foot is the basis. The acre based on the US Survey Foot is slightly larger by about 162 cm². Since 2023, the acre has also been based on the international foot in the United States.

Historical units

Although the size of the acres in the British Isles has been defined as 160 square rods since the High Middle Ages, its area was inconsistent depending on the place and time, since the unit of length rod or perch corresponded to different foot equivalents. It was only with the redefinition of the Imperial Units through the Weights and Measures Act of 1824 that a uniform acre was created for the entire British Empire.

Before the introduction of the Imperial Standard Acre (Statute Acre) there was, among other things, the old Scottish acre, the new Scottish acre, also known as Cunningham Acre, or the Irish or Plantation Acre. For example, the Cunningham Acre is about 1.3 times the size and the Plantation Acre roughly 1.6 times the size of today's acres. Some of these obsolete measures were in use well into the 20th century, for example in remote areas of Ireland.


  • Herbert Arthur Klein: The Science of Measurement. A Historical Survey. Dover Publications, Mineola NY 1988.
  • Johann Friedrich Krüger : Complete manual of the coins, measures and weights of all countries in the world. Gottfried Basse publishing house, Quedlinburg / Leipzig 1830.

Web links

Wiktionary: acre  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ^ Inflection of acre . Canoonet ; Retrieved September 23, 2019, and spelling Acre .; Retrieved December 22, 2012.
  2. Previously only used in the UK; see Weights and Measures & c. Act 1976 .
  3. Previously used especially in the United States; see Appendix C - General Tables of Units of Measurement. In: National Institute of Standards and Technology (Ed.): Handbook 44 - 2017 .
  4. Oxford English Dictionary , Volume I, page 117 f. - Cf. also Herbert Arthur Klein: The Science of Measurement. A Historical Survey. Dover Publications, Mineola NY 1988, p. 76.
  5. Federal Register (of the United States Government) of October 17, 2019: Deprecation of the United States (US) Survey Foot
  6. On Ireland cf. What is a Cunningham (Scottish) Acre .