Article — Square Miles to Square Kilometers
Square miles to square kilometers converter
One square mile equals 2.58998811 square kilometers, exactly. The conversion comes from the 1959 international definition of the statute mile: 1 mile = 1.609344 km, and squaring both sides gives 2.58998811 km² per mi².
This converter handles the math both directions and at any precision. Behind it sits a single exact factor — not an approximation, not a rounded engineering value, but a treaty number agreed in 1959 by the US, UK, Canada, Australia, New Zealand, and South Africa. Anything you read in km² for a US state or county was almost certainly derived from a square-mile measurement and this conversion.
What is 1 square mile in square kilometers?
One square mile equals about 2.59 km², or 2.58998811 km² to nine significant figures. For mental arithmetic, multiply square miles by 2.6 — that gives a result within 0.4% of the exact answer, which is good enough for road-map navigation and casual geography. For surveying, real estate, and GIS work where the area touches legal boundaries, use the full factor.
The reverse — square kilometers to square miles — uses the reciprocal: 0.3861022 mi² per km². For a quick estimate, divide by 2.6 (or multiply by 0.4). 1000 km² is just under 386 mi².
The 1999 Mars Climate Orbiter crashed because Lockheed Martin sent navigation thrust data in pound-force-seconds while NASA expected newton-seconds. A unit conversion error cost NASA $327 million and proved that even space agencies can fumble a square-miles-to-km² style mismatch when stakes are high.
The square miles to square kilometers formula
The factor is the square of the linear mile-to-km conversion. Linear miles convert at 1.609344 km/mi; areas are length squared, so the area conversion is 1.609344² = 2.58998811 km²/mi².
km² = mi² × 2.58998811 mi² = km² × 0.386102161 mi² = 2.59 km² 1 km² = 0.386 mi²1 mi² = 640 acres 1 mi² = 259 hectaresWhy the factor is 2.59 (not 1.61)
The single most common square-miles-to-km² error is multiplying area by the linear factor. The mistake gives an answer about 38% smaller than the correct value, and it surfaces in news graphics and government bulletins more often than it should.
The reason area squares the linear factor is that area itself is two-dimensional. Take a square 1 mile on each side. Each side is 1.609344 km long. The area is 1.609344 km × 1.609344 km = 2.589988 km². Not 1.609344 km², which would be the perimeter of one side, not the area enclosed.
Always check that the units on both sides match. mi² × 1.60934 gives km·mi — nonsense. mi² × 2.59 gives km², the right answer. The same rule applies to cubic conversions: mi³ × 1.60934³ = mi³ × 4.168 = km³.
Square miles, acres, and hectares
The square mile and the acre are tied by an exact 640-to-1 ratio. The acre originally meant the area a yoke of oxen could plow in a single day, formalized in medieval English law as a strip 1 furlong long by 1 chain wide (660 ft × 66 ft = 43,560 ft²). Forty-three thousand five hundred sixty multiplied by 640 yields 27,878,400 ft² — exactly one square mile.
The hectare is the metric counterpart, defined as 100 m × 100 m = 10,000 m² = 0.01 km². One square mile equals 258.999 hectares, so an acre is about 0.405 hectares. US Public Land Survey townships are 6 × 6 grids of sections, each 1 mi² (640 acres), totaling 36 mi² (23,040 acres) per township.
- 1 mi² = 2.58999 km² = 640 acres = 259 hectares
- 1 km² = 0.386 mi² = 247 acres = 100 hectares
- 1 acre = 0.00405 km² = 0.00156 mi²
- 1 township (US) = 36 mi² = 93.24 km²
- Manhattan = 22.83 mi² = 59.13 km²
- Singapore = 281 mi² = 728 km²
- Texas = 268,596 mi² = 695,662 km²
- Earth surface ≈ 197 million mi² = 510 million km²
US state sizes in square miles and km²
The US Census Bureau reports state areas in mi² because they are derived from the Public Land Survey System. International atlases convert to km² for consistency with the rest of the world. Both numbers describe the same land — only the convention differs.
California, the third-largest state, covers 163,696 mi² (423,970 km²). That alone is larger than Germany (357,022 km²) or the UK (244,376 km²). Texas, second behind Alaska, covers 268,596 mi² (695,662 km²), larger than any European country except Russia and Ukraine. Alaska at 665,384 mi² (1,723,337 km²) is bigger than Iran (1,648,195 km²).
Common square miles conversion mistakes
Beyond the linear-vs-area trap, three other errors keep appearing in published figures. The first is using nautical miles (1 nmi = 1.852 km) instead of statute miles (1 mi = 1.60934 km) for land areas. Nautical miles are for the sea and the air; land deeds and Census data are statute. The second is dropping the 640-to-1 acre ratio and writing "1 acre = 1 mi²", which understates land 640 times. The third is mixing US survey miles with international miles, a 2-parts-per-million difference that matters only in geodetic surveying.
When in doubt about which mile is meant, use the statute mile of 1.609344 km. That is the default for land area in the US, UK, and English-speaking countries, and the one this converter uses.
A short history of the statute mile
The Roman mile, mille passus, was 1000 paces — roughly 1.48 km. England's statute mile of 5280 feet was fixed by Elizabeth I in 1593 to align with the existing furlong (one-eighth of a mile, or 220 yards). Until 1959 the US survey mile and the UK statute mile differed slightly: the US survey foot was defined in 1893 as 1200/3937 meters, while the UK foot was 0.3048 meters. The two miles diverged by 3.2 mm per mile.
In 1959 the US, UK, Canada, Australia, New Zealand, and South Africa signed the International Yard and Pound Agreement, setting the international yard at exactly 0.9144 m and the international mile at 1609.344 m. The US Public Land Survey continued using the older "survey mile" for legacy deeds until 2022, when NIST formally retired the US survey foot. Today, one mile is one mile, and the conversion to square kilometers is one fixed number: 2.58998811.