How to convert inches to millimetres
One inch equals 25.4 millimetres, so multiply the number of inches by 25.4:
mm = in × 25.4
in = mm ÷ 25.4 (1 mm = 0.03937 in)
Example: 0.125 in → 0.125 × 25.4 = 3.175 mm.
Going the other way, divide by 25.4, or multiply by 0.03937; the millimetres to inches converter does it for you.
Everyday values
| Inches | Millimetres | For example |
|---|---|---|
| 0.125 in | 3.175 mm | 1/8 inch |
| 0.25 in | 6.35 mm | 1/4 inch |
| 0.375 in | 9.525 mm | 3/8 inch |
| 0.5 in | 12.7 mm | 1/2 inch |
| 0.75 in | 19.05 mm | 3/4 inch |
| 1 in | 25.4 mm | 1 inch |
When you need this conversion
Fractional inches are everywhere in hardware: drill bits, sockets, plywood thickness and plumbing. When you need a metric equivalent, multiply by 25.4. Socket and spanner sizes do not always have a perfect match: a 1/2-inch socket (12.7 mm) is close to 13 mm, while 3/8 inch (9.53 mm) sits between 9 and 10 mm.
Inch to millimetre conversion table
| in | mm | in | mm |
|---|---|---|---|
| 0.125 in | 3.175 mm | 1.625 in | 41.275 mm |
| 0.25 in | 6.35 mm | 1.75 in | 44.45 mm |
| 0.375 in | 9.525 mm | 1.875 in | 47.625 mm |
| 0.5 in | 12.7 mm | 2 in | 50.8 mm |
| 0.625 in | 15.875 mm | 2.125 in | 53.975 mm |
| 0.75 in | 19.05 mm | 2.25 in | 57.15 mm |
| 0.875 in | 22.225 mm | 2.375 in | 60.325 mm |
| 1 in | 25.4 mm | 2.5 in | 63.5 mm |
| 1.125 in | 28.575 mm | 2.625 in | 66.675 mm |
| 1.25 in | 31.75 mm | 2.75 in | 69.85 mm |
| 1.375 in | 34.925 mm | 2.875 in | 73.025 mm |
| 1.5 in | 38.1 mm | 3 in | 76.2 mm |
About the units
- Inch (in): exactly 2.54 centimetres (international agreement of 1959).
- Millimetre (mm): one thousandth of a metre.
Both are units of length. The conversion factor comes from the exact definitions above, so the calculator is exact; results are only rounded for display (up to six significant digits).
Frequently asked questions
How many millimetres are in one inch?
One inch is 25.4 millimetres. One millimetre is 0.03937 inches.
How do I convert inches to millimetres?
Multiply the value in inches by 25.4. To go back, divide by 25.4.
What is 0.125 in in millimetres?
0.125 in = 3.175 mm.
What is 0.25 in in millimetres?
0.25 in = 6.35 mm.
Sources
- NIST Special Publication 811 (2008), Guide for the Use of the International System of Units (SI), Appendix B: conversion factors.
- NIST Handbook 44 (2024), Appendix C: General tables of units of measurement.
Last reviewed 5 October 2026. Results are estimates; see our disclaimer.