The acre is a unit of land area used in the imperial and US customary systems. It is defined as the area of 1 chain by 1 furlong, lucia, St. Helena, St. Kitts and Nevis, St. Vincent and the Grenadines, Turks and Caicos, the United Kingdom, the United States and the US Virgin Islands. 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 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. An acre was defined in the Middle Ages, being the amount of land that could be ploughed in one day with a yoke of oxen. One acre equals 0.0015625 square miles,4,840 square yards,43,560 square feet or about 4,047 square metres. While all modern variants of the acre contain 4,840 square yards, there are definitions of a yard. A square enclosing one acre is approximately 69.57 yards, 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, consequently, the international acre is exactly 4,046.8564224 square metres. The US survey acre is about 4,046.872609874252 square metres, its value is based on an inch defined by 1 metre =39.37 inches exactly. Surveyors in the United States use both international and survey feet, and consequently, both varieties of acre. Since the difference between the US survey acre and international acre is only about a quarter of the size of an A4 sheet of paper, areas are seldom measured with sufficient accuracy for the different definitions to be detectable. In India, residential plots are measured in cents or decimal, in Sri Lanka the division of an acre into 160 perches or 4 roods is common. To be more exact, one acre is 90.75 percent of a 100 yards long by 53.33 yards wide American football field, the full field, including the end zones, covers approximately 1.32 acres. For residents of countries, the acre might be envisaged as approximately half of a 105 metres long by 68 metres wide association football pitch. It may also be remembered as 44,000 square feet, in English it was historically spelled aker. The acre was approximately the amount of land tillable by a yoke of oxen in one day and this explains one definition as the area of a rectangle with sides of length one chain and one furlong
Comparison of some Imperial and metric units of area
The area of one acre (red) superposed on an American football field (green) and association football (soccer) pitch (blue).