How this calculator works
A square mile is a square one mile on each side. The mile is exactly 1,609.344 m, so a square mile is 2,589,988.110336 square metres. The international acre is 4,046.8564224 square metres, and dividing one by the other gives exactly 640 acres per square mile. This is a definition, not an approximation.
Formula
square miles = acres / 640. acres = square miles x 640. square kilometres = square miles x 2.589988110336. hectares = square miles x 258.9988110336.
Worked example
640 acres = 1 square mile = 2.589988110336 km2 = 258.9988110336 ha. A 5,000 acre ranch is 7.8125 square miles, or about 20.2343 km2.
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Frequently asked questions
Why are there 640 acres in a square mile?
One square mile is 640 square chains, and an acre is ten square chains. The definitions of the mile, chain and acre fit together exactly, so the 640 figure is exact.
Is a section the same as a square mile?
In the US Public Land Survey System, a section is nominally one square mile, or 640 acres. Survey corrections mean real sections can differ slightly from a perfect square.
How do I compare this with hectares?
One square mile is about 259 hectares. One acre is about 0.404686 hectares, so a 640 acre square mile is 640 x 0.404686 = 258.9988 hectares.