Inches to Millimetres (in to mm)
1 in = 25.4 mm. Multiply inches by 25.4; the box below does it both ways.
millimetres
0.5 in = 12.7 mm · millimetres to inches page
How to convert inches to millimetres
- Take the value in inches.
- Multiply by 25.4 (because 1 in = 25.4 mm).
- Example: 0.5 in × 25.4 = 12.7 mm.
- Going back: divide by 25.4, or multiply by 0.03937008.
Where this comes up
Multiply inches by 25.4. Fractional inches are common on hardware and drill bits: ⅛ in is 3.175 mm, ¼ in is 6.35 mm, ½ in is 12.7 mm. The table lists those fractions as decimals so you can match a drill bit or bolt to the nearest metric size.
| in | mm |
|---|---|
| 0.125 | 3.175 |
| 0.25 | 6.35 |
| 0.375 | 9.525 |
| 0.5 | 12.7 |
| 0.625 | 15.875 |
| 0.75 | 19.05 |
| 1 | 25.4 |
| 1.5 | 38.1 |
| 2 | 50.8 |
| 3 | 76.2 |
| 4 | 101.6 |
| 6 | 152.4 |
| 12 | 304.8 |
Questions people ask
How many millimetres are in 1 inch?
1 in = 25.4 mm. To convert, multiply the inch value by 25.4.
What is 1 in in mm?
1 in is 25.4 mm.
How do I convert millimetres back to inches?
Divide by 25.4, or multiply by 0.03937008: 1 mm = 0.03937008 in.
More length conversions
Millimetres to Inches (mm to in) · m to ft · ft to m · cm to in · in to cm · cm to ft · ft to cm · ft to in · in to ft · cm to mm · mm to cm · all converters
Factors are the exact international definitions; last checked 2026-10-10.