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. The most common use of the acre is to measure tracts 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). 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 1/16th 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 comprises 4,840 square yards, 43,560 square feet[1] or about 4,050 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 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 U.S and in countries where the Imperial System is still in use. As of 2010, 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 x 660.

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