The square mile is a unit of area equal to 640 acres, or 2.589988 square kilometres. Its symbol is sq mi. It is the area of a square one mile on a side, and it is the unit in which American and British counties, cities, parks and disaster zones are described. Its equivalence to exactly 640 acres is not an accident but the foundation of how much of North America was laid out.
The United States Public Land Survey, begun in 1785, divided the western territories into townships of six miles square, each containing thirty-six sections of one square mile. A section holds 640 acres, a quarter section 160, and a quarter of that 40. Those numbers ran through homestead law, land sales and inheritance for a century, and they are still visible from the air in the grid of roads that follows section lines across the Midwest.
Because the survey made the square mile a legal container rather than a mere measurement, it shaped settlement patterns. A quarter section was considered the amount of land a family could work, so farms across the plains came in multiples of 160 acres, and the mile spacing of section-line roads set how far apart towns, schools and grain elevators could usefully sit.
Today its everyday use is descriptive. Population density in American statistics is people per square mile, so around 92 for the country as a whole against nearly 30,000 for Manhattan. Wildfire extent, flood inundation, national park size and metropolitan area are all reported the same way, and British usage matches it for county and national park areas even where other measurements have gone metric.
Converting is straightforward if the factor is remembered: one square mile is 2.59 square kilometres, and one square kilometre is 0.386 square miles. The relationship is not intuitive because it is the square of the length ratio, so a place described as ten times larger in square miles is only about 3.2 times larger in linear extent.
Its awkwardness shows when the three imperial land units are chained. A square mile is 640 acres, an acre is 4840 square yards, and a square yard is nine square feet, so going from a square mile to a square foot means multiplying by 27,878,400. That figure is exactly why land and building are measured with different units even inside the same system.
One square mile equals 640 acres, about 2.59 square kilometres, 259 hectares, or 27,878,400 square feet.
| Unit | Symbol | 1 Square mile equals | 1 of these equals | Converter |
|---|---|---|---|---|
| Square millimeter | mm² | 2589988110336 mm² | 3.86102158542 × 10-13 sq mi | Square miles to Square millimeters |
| Square centimeter | cm² | 25899881103.4 cm² | 3.86102158542 × 10-11 sq mi | Square miles to Square centimeters |
| Square decimeter | dm² | 258998811.034 dm² | 0.00000000386102158542 sq mi | Square miles to Square decimeters |
| Square meter | m² | 2589988.11034 m² | 0.000000386102158542 sq mi | Square miles to Square meters |
| Square decameter | dam² | 25899.8811034 dam² | 0.0000386102158542 sq mi | Square miles to Square decameters |
| Square hectometer | hm² | 258.998811034 hm² | 0.00386102158542 sq mi | Square miles to Square hectometers |
| Square kilometer | km² | 2.58998811034 km² | 0.386102158542 sq mi | Square miles to Square kilometers |
| Centiare | ca | 2589988.11034 ca | 0.000000386102158542 sq mi | Square miles to Centiares |
| Deciare | da | 258998.811034 da | 0.00000386102158542 sq mi | Square miles to Deciares |
| Are | a | 25899.8811034 a | 0.0000386102158542 sq mi | Square miles to Ares |
| Decare | daa | 2589.98811034 daa | 0.000386102158542 sq mi | Square miles to Decares |
| Hectare | ha | 258.998811034 ha | 0.00386102158542 sq mi | Square miles to Hectares |
| Square inch | sq in | 4014489600 sq in | 2.49097668605 × 10-10 sq mi | Square miles to Square inches |
| Square hand | sq hh | 250905600 sq hh | 0.00000000398556269768 sq mi | Square miles to Square hands |
| Square foot | sq ft | 27878400 sq ft | 0.0000000358700642792 sq mi | Square miles to Square feet |
| Square yard | sq yd | 3097600 sq yd | 0.000000322830578512 sq mi | Square miles to Square yards |
| Square chain | sq ch | 6400 sq ch | 0.00015625 sq mi | Square miles to Square chains |
| Acre | ac | 640 ac | 0.0015625 sq mi | Square miles to Acres |
| Square furlong | sq fur | 64 sq fur | 0.015625 sq mi | Square miles to Square furlongs |
| Square nautical mile | sq nmi | 0.755119708988 sq nmi | 1.32429333799 sq mi | Square miles to Square nautical miles |
| Square league | sq lea | 0.111111111111 sq lea | 9 sq mi | Square miles to Square leagues |