How this works
Pick a unit, type a value, and the calculator shows the equivalent in every other common length unit at the same time. The metric side covers millimetres up to kilometres; the imperial side covers inches up to miles, all using the international (1959) definitions adopted by NIST and ISO. Output precision adjusts automatically — large numbers get fewer decimals, very small ones get more — so you don't see "0.00" for sub-millimetre values.
The formula
Conversion factors used: 1 mm = 0.001 m, 1 cm = 0.01 m, 1 km = 1000 m, 1 in = 0.0254 m, 1 ft = 0.3048 m, 1 yd = 0.9144 m, 1 mi = 1609.344 m. Every conversion goes through metres as the canonical unit, so rounding error never compounds across multiple steps.
Example calculation
- Enter 5 km. The calculator first converts to metres: 5 × 1000 = 5,000 m.
- Then divides by each target factor: 5,000 / 0.0254 ≈ 196,850 inches; 5,000 / 0.3048 ≈ 16,404 feet.
- 5 km ≈ 3.1 miles — useful when an American friend asks how far your morning run is.
Frequently asked questions
Are these the international or the US-survey foot definitions?
International (1959) — 1 foot = 0.3048 metres exactly. The US-survey foot (used in some US land surveying) is about 2 ppm longer — the difference reaches a metre over a kilometre. The US officially retired the survey foot at the end of 2022, so you only see it on legacy GIS data and old deeds. For any modern use, the international foot is correct.
Why do I see scientific notation for very small numbers?
Below ~0.0001, decimal notation gets unwieldy fast — converting 1 mm into miles gives 0.000000621... which is hard to read at a glance. Scientific notation (6.214e-7) is more compact and preserves significant digits. Switch to a smaller source unit if you'd rather see plain decimals.
Can I convert between metric and astronomical / geographic units?
Not in this calculator yet — it covers the everyday metric and imperial units. Light-years, parsecs, nautical miles, fathoms, etc. are on a future roadmap. For now, do the metres conversion here and use a Wikipedia constant to bridge to the more exotic unit.
What's the difference between a yard and a metre?
1 yard = 0.9144 metres, so a metre is about 9.4% longer. Useful approximation: a yard ≈ a long stride; a metre ≈ that stride plus another inch. American football fields are 100 yards (≈ 91.44 m) end-to-end, which makes a "first down" exactly 10 yards = 9.144 m.