How Many Acres in a Square Mile?
Exactly 640 acres make up one square mile. The basis of the US land section system.
Last updated: 2026-05-21
There are 640 acres in 1 square mile.
A square mile measures 5,280 feet on each side. That gives 5,280 × 5,280 = 27,878,400 square feet. Divide by 43,560 (the number of square feet in one acre) and you get exactly 640 acres. The number is whole on purpose: both units descend from the same historic English surveying system, so they line up cleanly.
Square Miles to Acres Chart
| Square Miles | Acres |
|---|---|
| 0.25 | 160 |
| 0.5 | 320 |
| 1 | 640 |
| 2 | 1,280 |
| 5 | 3,200 |
| 10 | 6,400 |
| 50 | 32,000 |
| 100 | 64,000 |
To go the other way, divide acres by 640. For example, 1,000 acres ÷ 640 ≈ 1.5625 square miles.
The Section System
In US land surveying, the square mile is the foundation of how land is divided. Under the Public Land Survey System (PLSS), a section is exactly one square mile — 640 acres. Thirty-six sections make up a 6-mile by 6-mile township.
Sections are split into smaller, predictable parcels:
- Section — 1 square mile = 640 acres
- Half section — 320 acres
- Quarter section — 160 acres (the classic Homestead Act parcel)
- Quarter-quarter section — 40 acres (the “back forty”)
This is why so much of the rural United States is laid out on a tidy grid: roads and property lines follow section boundaries spaced one mile apart.
Other Units
The square mile converts cleanly into metric area units as well:
- 1 square mile = 2.58999 km²
- 1 square mile = 258.999 hectares
- 1 square mile = 27,878,400 ft²
- 1 acre ≈ 0.4047 hectares, so 640 acres ≈ 259 hectares
Need a different parcel size or unit? Use our area converter to switch between acres, square miles, hectares, and square kilometers instantly.