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The **square mile** (abbreviated as **sq mi** and sometimes as **mi²**)^{[1]} is an imperial and US unit of measure for an area equal to the area of a square with a side length of one statute mile.^{[2]} It should not be confused with *miles square*, which refers to a square region with each side having the specified length. For instance, 20 miles square (20 × 20 miles) has an area equal to 400 square miles; a rectangle of 10 × 40 miles likewise has an area of 400 square miles, but it is not 20 miles square.

One square mile is equal to:

- 4,014,489,600 square inches
^{[3]} - 27,878,400 square feet
^{[3]} - 3,097,600 square yards
^{[3]} - 640 acres
^{[1]} - 2560 roods
^{[4]}

Since one inch has been standardized to 2.54 cm by international agreement, a square mile is equivalent to the following metric measures:

- 25,899,881,103.36 square centimetres
- 2,589,988.110336 square metres
- 258.9988110336 hectares
- 2.589988110336 square kilometres

When applied to a portion of the earth's surface, "square mile" is an informal synonym for section.

Romans derived measurements from marching. Five feet was equal to one pace (which is appropriately a double step). One thousand paces measured a Roman mile, which was somewhat smaller than the English statute mile. This Roman system was adopted, with local variations, throughout Europe as the Roman Empire spread.^{[5]}

Total population in a square mile is derived by dividing the total number of residents by the number of square miles of land area in the specified geographic area. The population per square kilometer is derived by multiplying the population per square mile by 0.3861.^{[6]}

Land area measurements are originally recorded as whole square meters. Square meters are converted to square kilometers by dividing by 1,000,000; square kilometers are converted to square miles by dividing by 2.58999; square meters are converted to square miles by dividing by 2,589,988.^{[6]}

- ^
^{a}^{b}Rowlett, Russ (September 1, 2004). "S",*How Many? A Dictionary of Units of Measurement*. University of North Carolina at Chapel Hill. Retrieved February 22, 2012. **^**Davies, Charles (1872).*Mathematical dictionary and cyclopedia of mathematical science*. Original from Harvard University: A.S. Barnes and co. p. 582.- ^
^{a}^{b}^{c}François Cardarelli (2003).*Encyclopaedia of scientific units, weights, and measures: their SI equivalences and origins*. Springer. p. 3. ISBN 978-1-85233-682-0. Retrieved 22 February 2012. **^**Zupko, Ronald Edward (1985).*A dictionary of weights and measures for the British Isles: the Middle Ages to the twentieth century*. American Philosophical Society. p. 353. ISBN 978-0-87169-168-2. Retrieved 22 February 2012.**^**"The history of measurement". Retrieved 9 August 2015.- ^
^{a}^{b}"Land Area and Persons Per Square Mile".*Census.gov*. Retrieved 9 August 2015.^{[permanent dead link]}

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