Square miles to acres, or miles squared to acres.

To convert square miles to acres, multiply by 640. So 1 square mile is 640 acres, 2.5 square miles is 1,600 acres and a quarter square mile is 160 acres. The factor is exact. It comes from a mile being 5,280 feet: 5,280 × 5,280 = 27,878,400 square feet, and 27,878,400 ÷ 43,560 = 640.

How to use the converter

1. Type the area in the Square miles (mi²) box. The page opens on 1 square mile.
2. Read acres in the big result, then square kilometers, hectares and square feet in the cards.
3. If your source says "5 miles squared", read the next section before you type 5.
4. Press Copy result or Copy link to share the sum. The swap button (or S) turns acres into square miles.

"Miles squared" can mean two things

People who search for "miles squared to acres" usually mean square miles, and that is how this converter reads it. But in strict maths "5 miles squared" is 5 mi × 5 mi, an area of 25 square miles. Say a county is "5 miles squared": a square with sides of 5 miles has 16,000 acres, while 5 square miles is only 3,200 acres. Both readings appear in real writing. When it matters, ask whether the 5 is a side length or an area. For a side length, multiply the side by itself first, then type the result here. A square 6 miles by 6 miles is 36 square miles, or 23,040 acres.

The exact numbers

NIST Handbook 44 (2026), Appendix C (checked 26 Oct 2026) states that one square mile is 27,878,400 square feet and 640 acres, and gives it as 2,589,988.110 336 square meters with the international foot. That is 2.589988110336 km². The survey square mile is a little larger, 2,589,998.470 319 521 m², because it uses the retired survey foot. This page uses the international mile.

Worked examples

• 1/4 square mile. 0.25 × 640 = 160 acres. It is a square half a mile on each side.
• 1/2 square mile. 320 acres.
• 3 square miles. 1,920 acres.
• 10 square miles. 6,400 acres, which is 25.9 km².
• 100 square miles. 64,000 acres.

Square miles in metric

For a metric reader, 1 square mile is 2.59 km² or 259 ha. A hectare is 10,000 square meters, so one acre is 0.40469 ha. The cards under the result give the hectares and square kilometers for your own number.

Table

Where the sizes come up

Counties, parks, farms, reservoirs and military ranges are often described in square miles, while sale and tax records use acres. Moving between the two is a single multiplication, and it is worth doing the sum in both directions as a check. A farm of 1,920 acres is 3 square miles, because 3 × 640 = 1,920. The calculation works for any area on any scale: a pond of 0.002 square mile is 1.28 acres.

Common mistakes

Do not multiply miles by 640 when you have a length. 5 miles of road is not 3,200 acres. Do not mix up acres per square mile (640) with square feet per acre (43,560). And keep "square" with the unit: 1 mi is 5,280 ft, but 1 mi² is 27,878,400 ft².

The reverse is acres to square miles. For smaller plots see square feet to acres.