An acre is 43,560 square feet of area in any shape. This measure of square footage is not surface area, but is instead a measure along a flat plane. A square-shaped acre plot has 208.7 feet per side, and a square mile contains 640 acres.
The use of the international acre to measure land is predominant in the United States and U.S. territories, Antigua and Barbuda, The Bahamas, Belize, Dominica, Grenada, India, Jamaica, Montserrat, Myanmar, Pakistan, Samoa, St. Lucia, St. Kitts and Nevis, St. Vincent and the Grenadines, Turks and Caicos, and the United Kingdom and U.K. territories. The international acre is defined in terms of meters, but the United States also uses the survey acre, which is slightly different and is defined as exactly 43,560 square feet.