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.

One international acre is equal to 4,046.856 422 4 m exactly. One U.S. survey acre is equal to ⁄_15,499,969 m = 4,046.872 609 874 252 m approximately.

The area of one acre (red) overlaid on an American football field (green) and Association football field (blue).

One acre comprises 4,840 square yards or 43,560 square feet (which can be easily remembered as 44,000 square feet, less 1%; or as the product of 66 x 660). Because of alternative definitions of a yard or a foot, the exact size of an acre also varies slightly. 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 and 9 inches (63.6 meters) on a side. But as a unit of measure an acre has no prescribed configuration; any perimeter enclosing 43,560 square feet is an acre in size.

The acre is often used to express areas of land. In the metric system, the hectare is commonly used for the same purpose. An acre is approximately 40% of a hectare.

One acre is 90.75 percent of a 53.33-yard-wide American football field. The full field, including the end zones, covers approximately 1.32 acres (0.53 ha).