How this calculator works
Each unit is converted through the metre using the exact legal factors in NIST Special Publication 811. The international foot is exactly 0.3048 m, the yard is 3 ft and the mile is 5280 ft, so every unit pair uses a ratio of fixed factors. The readout uses six significant digits. The US survey foot, retired by NIST in 2023, is not used.
Formula
result = value × F(from) ÷ F(to), where F is the exact metres per unit: 1 in = 0.0254 m, 1 ft = 0.3048 m, 1 yd = 0.9144 m, 1 mi = 1609.344 m, 1 mm = 0.001 m, 1 cm = 0.01 m, 1 km = 1000 m.
Worked example
180 cm to inches: 180 × 0.01 ÷ 0.0254 = 70.8661 in. 2.5 mi to kilometres: 2.5 × 1609.344 ÷ 1000 = 4.02336 km.
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Frequently asked questions
Why is 1 inch exactly 2.54 centimetres?
The inch, foot, yard and mile have been defined in metric terms by international agreement since 1959, so the factors are legal definitions rather than measurements.
Is the US survey foot used?
No. NIST retired the US survey foot in 2023, and every calculation here uses the international foot of exactly 0.3048 m.
Can I convert straight to a feet plus inches readout?
Use the centimetres to feet and inches page, which splits the result into whole feet and the leftover inches.