Length & distance: the exact defined values
1 yard = 0.9144 metre exactly, and 1 inch = 25.4 mm exactly — both fixed by the 1959 international yard and pound agreement.
How the base unit is defined
The metre is defined by fixing the speed of light in vacuum at exactly 299 792 458 m/s (SI Brochure, 9th edition).
Exact values table
"Exact" means the value is fixed by definition and carries no measurement uncertainty.
| Quantity | Exact value | Notes |
|---|---|---|
| 1 inch | 0.0254 m (25.4 mm) | Exact by definition since 1959. Not a rounded measurement. Open converter → |
| 1 foot | 0.3048 m | 12 inches × 25.4 mm. Exact. Open converter → |
| 1 yard | 0.9144 m | 3 feet. The value NIST publishes as the international yard. Open converter → |
| 1 statute mile | 1609.344 m ≈ 1.609 km | 1760 yards × 0.9144 m. Open converter → |
| 1 nautical mile | 1852 m | Defined exactly by international agreement (1929), used in air and sea navigation. |
| 1 kilometre | 1000 m ≈ 0.621371 mile | SI prefix, exact by definition. Open converter → |
Rounding traps that actually change your result
The US survey foot is not the international foot
The US survey foot was 1200/3937 m (about 0.30480061 m) — roughly 2 parts per million longer than the international foot of 0.3048 m. NIST retired it for new work at the end of 2022, but old survey and land records still use it. Over a mile the difference is about 3 mm; over 100 km it is about 200 mm, which matters in surveying and geodesy and nowhere else.
1 inch = 2.54 cm is exact, not approximate
Many textbooks label it 'approximately'. It is not. Every inch-to-metric conversion on this site therefore carries no factor error at all — only display rounding, which we show to six decimal places on each conversion page.
Where these values come from
- NIST Handbook 44, Appendix C
Publishes the international yard as exactly 0.9144 m and the inch as exactly 25.4 mm.
- BIPM SI Brochure, 9th edition
Definition of the metre via the fixed speed of light.
- NIST Special Publication 811
Guide for the use of SI units and conversion factors.
Frequently asked questions
Is a yard exactly 0.9144 metres?
Yes. Under the 1959 international yard and pound agreement the yard is defined as exactly 0.9144 metre. It is a definition, so there is no measurement uncertainty attached to it.
Why is the inch exactly 25.4 mm?
The same 1959 agreement fixed the yard at 0.9144 m. An inch is 1/36 of a yard, so 0.9144 / 36 = 0.0254 m = 25.4 mm exactly.
How many metres are in a mile?
Exactly 1609.344 m. A statute mile is 1760 yards, and 1760 × 0.9144 = 1609.344.
Convert length & distance now
These converters use the exact factors listed above.