Square Kilometers to Square Miles

A kilometer is 0.621371 mi, but a square kilometer is not 0.621371 square miles. Area scales with the square of length, so the factor is 0.621371², about 0.386102. Use the plain length factor by mistake and the answer comes out 61 % too large.

The same rule holds for every area pair: square feet to square meters uses 0.3048², and square inches to square centimeters uses 2.54².

Geography figures are where this pair shows up most. National statistics in most of the world use km², while US sources quote square miles. Greater London covers about 1,572 km², roughly 607 sq mi, and Lake Superior's surface is about 82,100 km², or 31,700 sq mi.

Rounding matters less for areas than people expect, because geography figures are rarely precise to more than three or four digits; report the result with no more digits than the source had.

One square mile is exactly 640 acres, and one square kilometer is 100 hectares. Those pairs make it easy to move between map and land units: hectares to acres and acres to hectares cover smaller plots.

All of these use the international foot and mile defined in NIST SP 811, so the factors are exact rather than measured. For distances instead of areas, see km to miles.

Population density works the other way round. A city with 4,000 people per km² has about 10,360 people per square mile, because each square mile holds 2.59 km². The density figure goes up when converted to square miles, while the area figure goes down.