Acres to square miles, for large tracts of land.

To convert acres to square miles, divide the acres by 640. So 640 acres is 1 square mile, 160 acres is 0.25 square miles and 10,000 acres is 15.625 square miles. Dividing by 640 gives a finite decimal, so the answer is exact: 1 acre is exactly 0.0015625 square mile.

How to use the converter

1. Type the acreage in the Acres (ac) box. The page opens on 160 acres.
2. Read square miles in the big result. The cards give the same land in square kilometers, hectares and square meters.
3. For big numbers type digits without commas, or with them. Both are read.
4. Use Copy result, Copy link or Print chart. Click a row of Common values to load it, and press the swap button (or S) to go from square miles to acres.

Why 640

A square mile is 5,280 ft by 5,280 ft, or 27,878,400 square feet. An acre is 43,560 square feet. The quotient is 640, and NIST Handbook 44 (2026), Appendix C (checked 26 Oct 2026) states it directly: 1 square mile equals 640 acres. Because 640 is made only of twos and fives, dividing by it never repeats. That is why a half, a quarter and an eighth of a square mile are neat acre counts: 320, 160 and 80.

Picture the land

A square mile cut into quarters gives squares of 160 acres, each half a mile on a side. Cut again and you get 40 acre squares, a quarter of a mile on a side. The table gives the area for tracts that people ask about.

Worked examples

• 1,000 acres. 1,000 ÷ 640 = 1.5625 square miles.
• 5,000 acres. 7.8125 square miles.
• 100,000 acres. 156.25 square miles, or 404.7 km².
• 320 acres. Half a square mile, 0.5 square miles.
• 1 acre. 0.0015625 square miles.

A word on length

A square mile is an area. A tract that is 2 square miles is not 2 miles long. If it were a square it would be about 1.41 miles on each side, because the side is the square root of the area. For a tract 2 miles by 2 miles the area is 4 square miles, which is 2,560 acres. Always check whether a size is a side or an area before you convert.

Check by halving

Because 640 is 2 to the seventh power times 5, you can check an answer by halving. 640 acres is 1 square mile, 320 is a half, 160 a quarter, 80 an eighth, 40 a sixteenth and 20 a thirty-second. Any acreage that is one of those numbers is a power-of-two fraction of a square mile. 2,560 acres is 4 square miles, and 5,120 is 8.

In metric

One acre is 0.40469 ha, so 640 acres is 259 ha. A square mile is 2.59 km². These come from the international foot and are exact to the digits shown in the Handbook.

For the reverse see square miles to acres. For house lots use square feet to acres.