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Historical origin

The word acre is derived from Old English æcer originally meaning "open field", cognate with Norwegian åker, Icelandic akur, Swedish åker, German Acker, Dutch akker, Latin ager, Sanskrit ajr, and Greek αγρός (agros). In English, an obsolete variant spelling was aker.

According to the Act on the Composition of Yards and Perches, dating from around 1300, an acre is "40 perches in length and four in breadth", meaning 220 yards by 22 yards. As detailed in the diagram, an acre was roughly the amount of land tillable by a yoke of oxen in one day.

Before the enactment of the metric system, many countries in Europe used their own official acres. In France, the traditional unit of area was the arpent carré, a measure based on the Roman system of land measurement.The acre was used only in Normandy (and neighbouring places outside its traditional borders), but its value varied greatly across Normandy, ranging from 3,632 to 9,725 square metres, with 8,172 square metres being the most frequent value. But inside the same pays of Normandy, for instance in pays de Caux, the farmers (still in the 20th century) made the difference between the grande acre (68 ares, 66 centiares) and the petite acre (56 to 65 ca). The Normandy acre was usually divided in 4 vergées (roods) and 160 square perches, like the English acre.

The Normandy acre was equal to 1.6 arpents, the unit of area more commonly used in Northern France outside of Normandy. In Canada, the Paris arpent used in Quebec before the metric system was adopted is sometimes called "French acre" in English, even though the Paris arpent and the Normandy acre were two very different units of area in ancient France (the Paris arpent became the unit of area of French Canada, whereas the Normandy acre was never used in French Canada).

In Germany, the Netherlands, and Eastern Europe the traditional unit of area was Morgen. Like the acre, the morgen was a unit of ploughland, representing a strip that could be ploughed by one man and an ox or horse in a morning. There were many variants of the morgen, differing between the different German territories, ranging from 1 ⁄ 2 to 2 + 1 ⁄ 2 acres (2,000 to 10,100 m). It was also used in Old Prussia, in the Balkans, Norway, and Denmark, where it was equal to about two-thirds acre (2,700 m).

Statutory values for the acre were enacted in England, and subsequently the United Kingdom, by acts of:

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 1 ⁄ 2 mile (880 yards) and is 1 ⁄ 4 square mile in area, or 160 acres. These subunits are typically then again 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" refers 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.