# Square miles to Square kilometers (mi² to km²)

> Convert square miles to square kilometers: 1 mi² = 2.58999 km². Exact conversion factor, formula, 22-row table and worked examples, with JSON API and MCP access.

- Page id: `square-miles-to-square-km` · Quantity: Area (`area`, base unit m2) · Tool: `convert_units`
- Canonical page: https://tttkmbb.com/conversion/square-miles-to-square-km · This document: https://tttkmbb.com/conversion/square-miles-to-square-km.md · JSON: https://tttkmbb.com/conversion/square-miles-to-square-km.json (add `?value=` for a result)

## Formula

- `1 mi² = 2.58999 km²`
- `square kilometers = square miles × 2.58998811`
- Reverse: `square miles = square kilometers × 0.3861021585`

The factor follows from the SI definitions of both units (NIST SP 811); values below are rounded to 6 significant figures, the API returns up to 12.

## Worked examples

- 5 mi² = 12.9499 km²
- 12.5 mi² = 32.3749 km²
- 100 mi² = 258.999 km²

## Conversion table

| Square miles (mi²) | Square kilometers (km²) |
| --- | --- |
| 1 | 2.58999 |
| 2 | 5.17998 |
| 3 | 7.76996 |
| 4 | 10.36 |
| 5 | 12.9499 |
| 6 | 15.5399 |
| 7 | 18.1299 |
| 8 | 20.7199 |
| 9 | 23.3099 |
| 10 | 25.8999 |
| 15 | 38.8498 |
| 20 | 51.7998 |
| 25 | 64.7497 |
| 30 | 77.6996 |
| 40 | 103.6 |
| 50 | 129.499 |
| 75 | 194.249 |
| 100 | 258.999 |
| 200 | 517.998 |
| 250 | 647.497 |
| 500 | 1294.99 |
| 1000 | 2589.99 |

## API

- `GET https://tttkmbb.com/api/v1/convert?value=5&from_unit=mi2&to_unit=km2` → unified JSON envelope with `result.values.result`, `formula`, `factor`, `sources`, `freshness`
- MCP: server `https://tttkmbb.com/mcp`, tool `convert_units` with `{"value":5,"from_unit":"mi2","to_unit":"km2"}`
- All area units: https://tttkmbb.com/api/v1/units?quantity=area · full converter: [Area Converter](https://tttkmbb.com/conversion/area.md)
- Authentication: none. Freshness: `static` (fixed conversion factors).

## FAQ

**How many square kilometers are in one square mile?**

1 mi² = 2.58999 km². The factor comes from the SI definitions of both units (NIST SP 811 / BIPM), so it is exact or correct to at least 10 significant figures.

**What is the formula to convert square miles to square kilometers?**

Multiply the value in square miles by 2.58998811: square kilometers = square miles × 2.58998811. For example, 5 mi² = 12.9499 km².

**How do I convert square kilometers back to square miles?**

Multiply by the inverse factor 0.3861021585 (square miles = square kilometers × 0.3861021585), or use the square kilometers-to-square miles page.

## Related conversions

- [Square kilometers to square miles](https://tttkmbb.com/conversion/square-km-to-square-miles.md) (reverse)
- [Square meters to Square feet](https://tttkmbb.com/conversion/square-meters-to-square-feet.md)
- [Square feet to Square meters](https://tttkmbb.com/conversion/square-feet-to-square-meters.md)
- [Acres to Hectares](https://tttkmbb.com/conversion/acres-to-hectares.md)
- [Hectares to Acres](https://tttkmbb.com/conversion/hectares-to-acres.md)
- [Acres to Square meters](https://tttkmbb.com/conversion/acres-to-square-meters.md)
- [Square meters to Acres](https://tttkmbb.com/conversion/square-meters-to-acres.md)
- [Hectares to Square meters](https://tttkmbb.com/conversion/hectares-to-square-meters.md)
- [Square meters to Hectares](https://tttkmbb.com/conversion/square-meters-to-hectares.md)

## Data Sources

- NIST Special Publication 811 – Guide for the Use of the International System of Units (SI), Appendix B conversion factors — https://www.nist.gov/pml/special-publication-811 (standard, retrieved 2026-09-24)
- BIPM – The International System of Units (SI Brochure, 9th edition) — https://www.bipm.org/en/publications/si-brochure (standard, retrieved 2026-09-24)
- Wikipedia – Conversion of units — https://en.wikipedia.org/wiki/Conversion_of_units (reference, retrieved 2026-09-24)