Convert inch to millimetre
1 inch = 25.4 millimetres. Enter any value below for an exact conversion.
1 in =
25.4 mm
millimetres
Need the reverse? Convert millimetre to inch →
Formula
1 inch = 25.4 millimetres millimetre = inches × 25.4 inch = millimetres × 0.03937008
inch to millimetre conversion table
About converting inches to millimetres
Inches to millimetres is common in manufacturing and DIY, where imperial drawings need millimetre precision. Multiply inches by 25.4 to get an exact millimetre value.
Method & sources
Conversions use exact legal definitions rather than rounded approximations: 1 inch = 2.54 cm and 1 pound = 0.45359237 kg (fixed by the 1959 international yard and pound agreement), 1 mile = 1.609344 km, and 1 US liquid gallon = 3.785411784 litres. Temperatures convert with °F = °C × 9⁄5 + 32 and K = °C + 273.15. Results are rounded only for display.
- NIST Special Publication 811 — Guide for the Use of the SI
- NIST Handbook 44, Appendix C — General Tables of Units of Measurement
Page last updated: October 1, 2026
Frequently asked questions
How many millimetres are in a inch?
One inch equals 25.4 millimetres. So 10 inches is 254 millimetres. Use the converter above for any value.
What is the formula to convert inches to millimetres?
1 inch = 25.4 millimetres millimetre = inches × 25.4 inch = millimetres × 0.03937008
Is the inch-to-millimetre conversion exact?
Yes. The conversion factor between inches and millimetres is defined exactly, so results are precise — only rounded for display.
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