Square Miles to Acres
An instant square miles-to-acres converter, with the formula and a reference table below.
| Square Miles | Acres |
|---|---|
| 1 mi² | 640 ac |
| 2 mi² | 1280 ac |
| 3 mi² | 1920 ac |
| 5 mi² | 3200 ac |
| 10 mi² | 6400 ac |
| 25 mi² | 16000 ac |
| 50 mi² | 32000 ac |
| 100 mi² | 64000 ac |
How to use
- Enter a value in the Square Miles field.
- The Acres figure appears immediately — no button to press.
- Typing in the Acres field converts back to Square Miles the same way.
- Use the reference table for the values that come up most often.
Square Miles to Acres formula
To convert square miles to acres, multiply by 640.
ac = mi² × 640
Put another way, one square mile is exactly 640 acres.
Why this factor: the square mile and the acre are both defined against the square meter, so dividing one definition by the other gives 640 directly. Both definitions are set out in full below.
To go the other way, divide by 640 — or use the Acres to Square Miles converter, which is the same calculation reversed.
Worked example
Take 250 mi² and multiply by the factor 640: the result is 160000 ac.
250 mi² × 640 = 160000 ac
The converter above does this for any value you type, in either direction.
What is a square mile?
A square mile is the area of a one-mile square, used for large land areas in the US and UK.
Definition: Exactly 2,589,988.110336 square meters, and exactly 640 acres.
Where it is used: County, state and metropolitan areas, and population density in the US.
What is an acre?
An acre is the customary land area unit of the US and UK, historically a day's ploughing for one team of oxen.
Definition: One chain by one furlong — 43,560 square feet — which is exactly 4,046.8564224 square meters.
Where it is used: US and UK property and farmland, development parcels, and land sales.
Worth knowing: 640 acres make one square mile, the basis of the US Public Land Survey grid.
Square Miles vs Acres
The square mile and the acre belong to the same family of units, so converting between them is usually a matter of shifting a decimal point or applying a small whole-number factor.
Frequently asked questions
How many acres are in a square mile?
One square mile equals 640 acres.
What is the formula to convert square miles to acres?
Multiply the number of square miles by 640. To reverse it, divide by 640 instead.
How do I convert acres back to square miles?
Divide by 640, which is the same as multiplying by 0.0015625.
How many acres are in 10 square miles?
10 mi² works out to 6400 ac.
Is there anything to watch out for with acres?
640 acres make one square mile, the basis of the US Public Land Survey grid.
How is the square mile defined?
Exactly 2,589,988.110336 square meters, and exactly 640 acres.