Square Miles to Acres Converter - Convert mi² to acres (Exact: 1 mi² = 640 acres)
Accurate square miles (mi²) to acres converter using exact US customary definitions: 1 square mile = 640 acres. Ideal for land planning, agriculture, forestry, conservation, appraisal, and GIS reporting. Includes exact formulas, worked examples, expanded tables, rounding guidance, detailed FAQs, and practical tips.
Exact factor: 1 mi² = 640 acres ⇒ acres = mi² × 640. See all land area calculators.
About Square Miles to Acres Conversion
The square mile (mi²) is a regional-scale unit for counties, watersheds, and large management zones, while the acre (ac) is a parcel- and field-scale unit common in U.S. land markets, ranching, and many public datasets. Converting mi² to acres bridges top-level planning with on-the-ground reporting, letting your dashboards speak both languages without sacrificing accuracy.
Because 1 mi² = 640 acres is exact, your conversions are deterministic and audit-friendly across GIS, spreadsheets, app dashboards, and legal exhibits. A robust workflow stores areas in m², derives mi², acres, hectares, and km² only on render, and rounds once at presentation to avoid subtle drift between PDFs, emails, and CSV exports.
Typical scenarios include county-scale conservation summaries published in acres for U.S. audiences, translating watershed extents into parcel-comprehensible numbers, and reconciling legacy acre-based datasets with modern SI analytics.
Square Miles to Acres Formula
Exact relationship
Use either expression:
acres = mi² × 640
// inverse
mi² = acres ÷ 640Example:
1.25 mi² × 640 = 800 acresRelated Area Converters
What is a Square Mile (mi²)?
A square mile is the area of a square one mile on a side. Using SI reconciliation, 1 mi = 1609.344 m, so 1 mi² = (1609.344 m)² = 2,589,988.110336 m². This equals exactly 640 acres when divided by the international acre (4,046.8564224 m²). The unit is convenient for regional planning where acre totals become very large numbers.
What is an Acre (ac)?
The international acre is defined as exactly 4,046.8564224 m². It is the common language of U.S. parcels, agricultural fields, ranches, and timber stands. Publishing results in acres improves comprehension for many U.S. stakeholders while your backend can remain in SI for consistency with global metrics.
Step-by-Step: Converting mi² to acres
- Calculate or read the area in square miles (mi²) from your GIS or report.
- Multiply by 640 to convert to acres.
- Round once at presentation (e.g., 2–3 decimals for dashboards; follow legal/standard requirements for filings).
Example walkthrough:
Input: 0.375 mi²
Compute: 0.375 × 640 = 240 acres
Output: 240 acres (UI policy: whole acres or 1 decimal; maintain full precision internally)Common Conversions
Everyday quick checks (mi² → acres)
| mi² | acres | mi² | acres |
|---|---|---|---|
| 0.10 | 64 | 1.00 | 640 |
| 0.25 | 160 | 2.50 | 1,600 |
| 0.50 | 320 | 5.00 | 3,200 |
| 0.75 | 480 | 10.00 | 6,400 |
| 0.90 | 576 | 25.00 | 16,000 |
Quick Reference Table
Acres to square miles (ac → mi²)
| acres | mi² | acres | mi² |
|---|---|---|---|
| 100 | 0.15625 | 2,500 | 3.90625 |
| 160 | 0.25 | 5,000 | 7.8125 |
| 320 | 0.5 | 10,000 | 15.625 |
| 640 | 1 | 25,600 | 40 |
| 1,600 | 2.5 | 64,000 | 100 |
Precision, Rounding & Significant Figures
Operational rounding
For public dashboards, whole acres or 1–2 decimals are readable; for regulatory filings, match the survey or GIS method’s uncertainty. Always compute with full precision and round once at presentation to keep maps, PDFs, and exports synchronized.
Consistent documentation
Standardize field names (e.g., area_m2, area_km2, area_mi2, area_ac) and include a short methods note: “Exact constants; 1 mi = 1609.344 m; 1 ac = 4046.8564224 m²; 1 mi² = 640 ac; round once at presentation.” This prevents subtle drift during audits and reconciliations.
Where This Converter Is Used
- 🌾 Agriculture: County-level extents translated to acre totals for planning and grants.
- 🌳 Forestry & conservation: Fire perimeters and habitat extents converted to acres for public briefings.
- 🏞️ Parks & recreation: Master plans that summarize regional areas in mi² and publish parcels in acres.
- 🏡 Real estate & appraisal: Large holdings summarized in mi² but marketed in acres for familiar scale.
- 🛰️ GIS & remote sensing: Raster and vector rollups in m² converted to acres and mi² for dashboards.
Frequently Asked Questions
What is the exact formula to convert square miles to acres?
By US customary definition, one square mile equals exactly 640 acres. Therefore acres = mi² × 640 (exact). The inverse is mi² = acres ÷ 640 (exact). This identity holds whether you reconcile via meters (1 mi = 1609.344 m; 1 acre = 4046.8564224 m²) or stay entirely in customary units.
International vs. U.S. survey units-does 1 mi² still equal 640 acres?
Yes. Even if you reconcile through SI units (1 mi = 1609.344 m and 1 acre = 4046.8564224 m²), the ratio evaluates to 640 exactly. Legacy U.S. survey foot topics are largely historical now; modern federal standards use the international foot, but the 640 relation remains exact for land work and publishing.
Should I store land area in mi², acres, hectares, or m²?
Pick one canonical store-most teams choose square meters (m²) for analytics and interoperability. Convert to mi² and acres for U.S.-facing reports and to hectares/kilometers for international audiences. Converting at the edges, with one rounding step at presentation, keeps PDFs, maps, and CSVs in sync.
How should I round mi² ↔ acres for dashboards, listings, and filings?
Compute with full precision and round once at output. For public dashboards, 2–3 decimals in acres or 3–4 decimals in mi² are common. For survey annexes and compliance, follow your measurement uncertainty and governing standard. Publish a single rounding policy and apply it everywhere.
Do map projections change the conversion between mi² and acres?
No. Projections matter when you calculate the polygon’s area from geographic coordinates. Once the area is in mi² or m², converting to acres uses fixed unit definitions (mi² → acres is always ×640).
How do square miles relate to square kilometers and hectares?
1 mi² ≈ 2.589988110336 km² exactly from SI definitions. Since 1 km² = 100 hectares (ha), you get 1 mi² ≈ 258.9988110336 ha. Via acres, 1 ha ≈ 2.4710538147 acres, so bridges among these units are straightforward and exact through m².
What field names and documentation prevent unit mistakes?
Use explicit columns like area_m2, area_km2, area_mi2, area_ac, and area_ha. Add a methods note: “Exact constants: 1 mi² = 640 ac; 1 mi = 1609.344 m; 1 ac = 4046.8564224 m²; round once at presentation.” This avoids double rounding and off-by-factor errors across systems.
What anchor values help with quick sanity checks?
Memorize: 0.1 mi² = 64 ac; 0.25 mi² = 160 ac; 0.5 mi² = 320 ac; 1 mi² = 640 ac; 2.5 mi² = 1600 ac; 10 mi² = 6400 ac. For reverse checks: 100 ac = 0.15625 mi²; 640 ac = 1 mi².
Tips for Working with mi² & acres
- Keep m² canonical; derive mi², acres, ha, and km² for UI and reports.
- Round once at output; avoid re-rounding in exports and multi-step pipelines.
- Use explicit unit columns and publish constants in your documentation to prevent conversion drift.
- Maintain regression pairs (e.g., 1.25 mi² ↔ 800 ac; 100 ac ↔ 0.15625 mi²) and test both directions.