Area & land measurement: the exact defined values
1 acre = 4046.856 422 4 square metres exactly, because an acre is 4840 square yards and a yard is exactly 0.9144 m.
How the base unit is defined
The square metre derives directly from the metre, which is fixed by the speed of light.
Exact values table
"Exact" means the value is fixed by definition and carries no measurement uncertainty.
| Quantity | Exact value | Notes |
|---|---|---|
| 1 acre | 4046.8564224 m² ≈ 0.4047 hectare | 4840 yd² × 0.9144² . Exact. Open converter → |
| 1 hectare | 10000 m² ≈ 2.4711 acres | Exact by definition; the standard land unit outside the US. Open converter → |
| 1 square foot | 0.09290304 m² | 0.3048² . Exact. Open converter → |
| 1 square mile | 2589988.110336 m² = 640 acres | 1609.344² . Exact. |
| 1 square kilometre | 1000000 m² = 100 hectares | Exact. Open converter → |
Rounding traps that actually change your result
The US survey acre differs slightly from the international acre
Land titles recorded using the US survey foot give an acre of about 4046.87261 m² — roughly 0.0004% larger than the international acre of 4046.8564224 m². On a single residential plot this is a few square centimetres. On a large ranch it can be several square metres, which is why legal descriptions state which foot was used.
Where these values come from
- NIST Handbook 44, Appendix C
International acre as 4046.8564224 m², derived from the international yard.
- BIPM SI Brochure, 9th edition
The hectare as a non-SI unit accepted for land area.
Frequently asked questions
How many square metres is an acre?
Exactly 4046.8564224 m². An acre is 4840 square yards and a yard is exactly 0.9144 m, so 4840 × 0.9144² = 4046.8564224.
How many acres in a hectare?
One hectare is 10 000 m², which is 10000 / 4046.8564224 = 2.471053815... acres. This one does not terminate, so it is always shown rounded.
Convert area & land measurement now
These converters use the exact factors listed above.