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Acres to square miles: exact 1/640 factor, township grid context, charts, and land tips
Conversion formula
Verification: factors follow standard unit definitions; round for display only.
Quick reference chart
| Acre | Square Mile |
|---|---|
| 1 | 0.001563 |
| 2 | 0.003125 |
| 3 | 0.004687 |
| 4 | 0.00625 |
| 5 | 0.007813 |
| 6 | 0.009375 |
Educational explanation
Acres to square miles
Convert acres to square miles when a deed, tax assessor record, farm report, or zoning map lists land in acres but county atlases, mining claims, or regional planning summaries quote square miles.
The US survey system defines exactly 640 acres per square mile—one township section. Because 1 acre = 4046.8564224 m² and 1 sq mi = 2,589,988.110336 m² in the catalog:
square miles = acres × 0.0015625 · equivalently sq mi = acres ÷640
The fraction 1/640 is exact—not an approximation. Five acres = 5 ÷ 640 = 0.0078125 sq mi.
Step-by-step: convert 640 acres to square miles
- Write: sq mi = 640 ÷ 640.
- Result: 1 sq mi (one full section).
- Context: standard Public Land Survey System section in many US states.
Step-by-step: convert 40 acres to square miles
- 40 ÷ 640 = 0.0625 sq mi (1/16 sq mi).
- Known as a quarter-quarter section in homestead references.
Acres to square miles conversion chart
| Acres | Square miles (sq mi) | Typical context |
|---|---|---|
| 1 acre | 0.0015625 sq mi | Small pasture, definition unit |
| 5 acres | 0.0078125 sq mi | Rural homesite, hobby farm |
| 10 acres | 0.015625 sq mi | Small orchard block |
| 40 acres | 0.0625 sq mi | Quarter-quarter section |
| 80 acres | 0.125 sq mi | Quarter section (half mile × half mile) |
| 160 acres | 0.25 sq mi | Quarter section (homestead reference) |
| 320 acres | 0.5 sq mi | Half section |
| 640 acres | 1 sq mi | Full section (definition anchor) |
Quick mental estimate (no calculator)
Divide acres by 640—exact. Memorize anchors: 40 ac = 1/16 sq mi, 160 ac = 1/4 sq mi, 640 ac = 1 sq mi.
Square miles to acres
Convert square miles to acres when a county atlas, regional GIS layer, or mining tenure map lists square miles but deeds, USDA forms, or rural MLS entries expect acres.
Multiply square miles by 640 to obtain acres. This is the exact US township-grid relationship.
acres = square miles × 640
Step-by-step: convert 1 square mile to acres
- 1 × 640 = 640 acres.
- Equals 27,878,400 sq ft (640 × 43,560).
Step-by-step: convert 0.25 square mile to acres
- 0.25 × 640 = 160 acres (quarter section).
Square miles to acres conversion chart
| Square miles (sq mi) | Acres | Typical context |
|---|---|---|
| 0.0015625 sq mi | 1 acre | Reverse anchor |
| 0.015625 sq mi | 10 acres | Small commercial tract |
| 0.0625 sq mi | 40 acres | Quarter-quarter section |
| 0.25 sq mi | 160 acres | Quarter section |
| 0.5 sq mi | 320 acres | Half section |
| 1 sq mi | 640 acres | Full section |
| 2 sq mi | 1280 acres | Two sections |
| 5 sq mi | 3200 acres | Small township portion |
Township grid, deeds, and related tools
640 acres per square mile is exact by US survey definition. Use it for section-scale land; use acres-to-square-feet for parcel building coverage.
Public Land Survey System
Townships are 6 miles × 6 miles = 36 sq mi = 23,040 acres. Sections within are 1 sq mi = 640 ac. For sq ft parcel math, see acres to square feet.
Real estate and agriculture
Listings may quote 5 ac while county summaries aggregate by sq mi. Convert before comparing to regional averages. For metric land, chain through acres to hectares.
Common mistakes to avoid
- Using 640 as approximate. It is the exact definition for US survey sections.
- Confusing linear miles with square miles. A 1-mile road ≠ 1 sq mi of land.
- Mixing acres with hectares in sq mi totals. Convert to one unit first.
- Assuming all countries use 640 ac/sq mi. Verify jurisdiction on foreign cadastral maps.
Related area converters
Nearby workflows: square miles to acres, acres to square feet, hectares to square miles, and square miles to square feet.
Frequently asked questions
How do I convert acres to square miles?
Divide acres by 640. Example: 160 acres ÷ 640 = 0.25 sq mi.
How do I convert square miles to acres?
Multiply square miles by 640. Example: 2 sq mi × 640 = 1280 acres.
How many square miles is one acre?
Exactly 1/640 sq mi = 0.0015625 sq mi.
How many acres are in one square mile?
Exactly 640 acres. This is the US survey section definition.
How many square miles is 40 acres?
40 ÷ 640 = 0.0625 sq mi (one quarter-quarter section).
Is the acre-to-square-mile conversion exact?
Yes. 640 acres × 4046.8564224 m²/acre = 2,589,988.110336 m² = 1 sq mi exactly at catalog precision.
How many acres is 0.5 square mile?
0.5 × 640 = 320 acres (half section).
How does 1 sq mi compare to hectares?
1 sq mi ≈ 259 ha. Use hectare-to-square-mile for that conversion path.
What is a good round-trip check?
Convert 640 ac → 1 sq mi → 640 ac.
Why is 640 acres called a section?
The US Public Land Survey System divides townships into 36 one-square-mile sections, each 640 acres.
Can I convert acres to sq mi through square feet?
Yes: acres × 43,560 = sq ft, then sq ft ÷ 27,878,400 = sq mi. Dividing by 640 is simpler.
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