The acre is commonly used in many countries, including the United Kingdom, the United States, Canada, India, Ghana, Liberia, and others.
The international symbol of the acre is ac. The most commonly used acre today is the international acre. In the United States both the international acre and the US survey acre are in use, but differ by only two parts per million; see below. The most common use of the acre is to measure tracts of land. One international acre is defined as exactly 4,046.8564224 square metres.
One acre equals 0.0015625 square miles, 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.
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. Consequently, the international acre is exactly 4,046.8564224 square metres.