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A square mile (symbol sq. mi. or mi²) is an imperial unit which is the area of a square whose side is one mile (or 5,280 feet). A square mile is equal to 27,878,400 square feet, 640 acres, or roughly 2½ square kilometres (2.589988 km², to be more exact). Note that the abbreviation sq. mi. is more common than mi², as the use of a superscript 2 to indicate a squared unit is more common with SI units.
See also: Conversion of units