The square mile (abbreviated as sq mi and sometimes as mi) is an imperial and US unit of measure for an area equal to the area of a square of one statute mile. It should not be confused with miles square, which refers to the number of miles on each side squared. For instance, 20 miles square (20 × 20 miles) is equal to 400 square miles. One square mile is equivalent to: 4,014,489,600 square inches 27,878,400 square feet, 3,097,600 square yards, 640 acres, 258.9988110336 hectares, 2560 roods, 25,899,881,103.36 square centimetres, 2,589,988.110336 square metres, 2.589988110336 square kilometres When applied to a portion of the earth's surface, which is curved rather than flat, 'square mile' is an informal synonym for section.