A square mile is 640 acres.
Square miles to Acres Converter — mi2 to ac
Convert Square mile (mi2) to Acre (ac) using the exact conversion factor (1 square mile = 640 acre). See the formula, worked examples, conversion table, unit comparison, history, and common uses.
Square miles to Acres converter
Converter
Area Converter
Convert land, construction, mapping, survey, and scientific area units.
Context values
Some unit families need one extra value for a physically accurate result.
An international acre is 4,046.8564224 square meters.
Result = input x 2.58998811e+6 / 4046.856422.
Area uses the square meter as the base unit.
Reserved above the conversion cards and below the converter.
About Converting Square miles to Acres
Comparing Square mile and Acre is common in agriculture and real estate, where US land is typically measured in acres and most of the rest of the world uses hectares. Square mile is typically associated with united States (county, metro, and regional area reporting), while Acre is typically associated with united States.
Formula
acres = square miles × 640
This factor comes from each unit's defined relationship to the category's base unit: 1 square mile equals 2589988.11034 base units, and 1 acre equals 4046.8564224 base units, so dividing one by the other gives the direct square mile-to-acre factor of 640.
Simple example
1 mi2 × 640 = 640 ac
1 square mile = 640 acres.
Real-world example
10 mi2 × 640 = 6,400 ac
10 square miles = 6,400 acres.
Conversion table
| Square mile (mi2) | Acre (ac) |
|---|---|
| 1 mi2 | 640 ac |
| 5 mi2 | 3,200 ac |
| 10 mi2 | 6,400 ac |
| 25 mi2 | 16,000 ac |
| 50 mi2 | 32,000 ac |
| 100 mi2 | 64,000 ac |
Reverse conversion: Acre to Square mile
640 ac × 0.0015625 = 1 mi2
640 acres = 1 square mile.
square miles = acres × 0.0015625
For a page dedicated to this direction, see Acre to Square mile.
Square mile vs. Acre
| Aspect | Square mile | Acre |
|---|---|---|
| Family | US/imperial customary unit | US/imperial customary unit |
| Typical use | US city, county, and regional area statistics | Measuring farmland, ranch, and rural property size in the US |
| Where it's used | United States (county, metro, and regional area reporting) | United States |
Understanding the Square mile
Square mile (mi2) measures area. A square mile is exactly 2,589,988.110336 square meters, derived from the international mile fixed in 1959. It is classified as a US/imperial customary unit.
Where it's used: United States (county, metro, and regional area reporting)
Uses of the Square mile today
- US city, county, and regional area statistics
- Population density reporting (people per square mile)
- Land-use and geography in US contexts
History of the Square mile
The square mile follows directly from the statute mile, itself fixed at exactly 1,609.344 meters by the 1959 International Yard and Pound Agreement; squaring that value gives the modern square mile.
Historical origin. Square mile was not the invention of a single named person; it developed through the process described above.
Understanding the Acre
Acre (ac) measures area. The international acre is defined as exactly 4,046.8564224 square meters. It is classified as a US/imperial customary unit.
Where it's used: United States United Kingdom (informal/historical, land registry legacy) Countries with historical British land-survey systems
International acre vs. US survey acre. The US survey acre, based on the US survey foot, differs from the international acre by about two parts per million. The US retired the survey foot/acre for federal use at the end of 2022.
Uses of the Acre today
- Measuring farmland, ranch, and rural property size in the US
- Real estate listings and zoning for land parcels
- Forestry and land-management reporting
History of the Acre
The acre originated in medieval England as a practical field measurement: the area a yoke of oxen could plow in one day, roughly a furlong (660 ft) long by a chain (66 ft) wide, giving the 43,560 square feet still used today.
Edward I's 1305 statute formalized this figure. In 1959, the same International Yard and Pound Agreement that fixed the pound also fixed the international acre at exactly 4,046.8564224 m², though the US separately retained a slightly different 'US survey acre' for some legacy land records until it was phased out in 2023.
Historical origin. Acre was not the invention of a single named person; it developed through the process described above.
Square mile — sources
- NIST — International yard and pound (1959) — Basis for the modern mile and its derived area unit.
Acre — sources
- NIST — US Survey Foot: Revised Unit Conversion Factors — Documents the 1959 international acre definition and the 2022/2023 survey-foot retirement.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many acres are in 1 square mile?
1 square mile equals 640 acres, using the exact defined conversion factor rather than an estimate.
How do I convert square mile to acre?
Multiply the square mile value by 640. The converter above does this instantly to whatever precision you set.
How do I convert acre back to square mile?
Use the reverse factor: 1 acre equals 0.0015625 square miles. You can also use the swap control in the converter above to flip the direction instantly.
What is a square mile?
Square mile (mi2) measures area. A square mile is exactly 2,589,988.110336 square meters, derived from the international mile fixed in 1959. It is classified as a US/imperial customary unit.
What is a acre?
Acre (ac) measures area. The international acre is defined as exactly 4,046.8564224 square meters. It is classified as a US/imperial customary unit.
Is the square mile to acre conversion exact?
Yes. The international acre is defined as exactly 4,046.8564224 square meters. That fixed definition is what the converter above uses, so any rounding you see is a display choice, not a limitation of the math.
What is the difference between international acre vs. us survey acre?
The US survey acre, based on the US survey foot, differs from the international acre by about two parts per million. The US retired the survey foot/acre for federal use at the end of 2022.
Where is the square mile still used today?
United States (county, metro, and regional area reporting)