How this calculator works
The international inch is exactly 25.4 mm, so a metric length in millimetres becomes inches by dividing by 25.4. Centimetres, metres and kilometres are decimal multiples of the millimetre, so the same inch factor works once the value is in millimetres. Drawings that mix metric dimensions with fractional inch hardware are a common source of confusion: a 6 mm bolt is not the same as a 1/4 inch bolt, even though the numbers look close.
Formula
in = mm / 25.4. mm = in x 25.4. For other metric units, first convert to millimetres: 1 cm = 10 mm, 1 m = 1,000 mm and 1 km = 1,000,000 mm.
Worked example
25 mm = 0.984252 in. 10 mm = 0.393701 in. 12 mm = 0.472441 in. 19 mm = 0.748031 in. 50 mm = 1.96850 in. 1 in = 25.4 mm.
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Frequently asked questions
How many inches is 10 mm?
10 mm is 0.393701 in, a little under 13/32 in. It is not 3/8 in, which is 9.525 mm. The nearest common fractional size depends on the tolerance you can accept.
Is 25 mm the same as one inch?
No. One inch is exactly 25.4 mm, so 25 mm is 0.984252 in, about 0.4 percent short. For many purposes they are close, but they are not interchangeable where a fit matters.
How do I convert centimetres to inches?
Divide by 2.54. Since 1 cm is 10 mm and 1 in is 25.4 mm, 1 cm is 0.393701 in. Choose centimetres as the source unit on this page to do it directly.
Why is an inch defined as 25.4 mm?
The 1959 international agreement on the yard and the pound fixed the inch at exactly 25.4 mm, ending small differences between national standards. The definition is exact and has no uncertainty.