1 inch = 25.4 millimetres
Precision engineering usually works in millimetres even in imperial countries, because 25.4 is exact and gives three decimal places of resolution without fractions.
Formula mm = in × 25.4
Type in either field — the other updates on every keystroke. Need other units? Open the full length converter.
| Inch | Millimetre |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 4 in | 101.6 mm |
| 5 in | 127 mm |
| 10 in | 254 mm |
| 15 in | 381 mm |
| 20 in | 508 mm |
| 25 in | 635 mm |
| 50 in | 1270 mm |
| 100 in | 2540 mm |
| 500 in | 12700 mm |
| 1000 in | 25400 mm |
| Millimetre | Inch |
|---|---|
| 1 mm | 0.03937 in |
| 2 mm | 0.07874 in |
| 3 mm | 0.11811 in |
| 4 mm | 0.15748 in |
| 5 mm | 0.19685 in |
| 10 mm | 0.393701 in |
| 15 mm | 0.590551 in |
| 20 mm | 0.787402 in |
| 25 mm | 0.984252 in |
| 50 mm | 1.968504 in |
| 100 mm | 3.937008 in |
| 500 mm | 19.685039 in |
| 1000 mm | 39.370079 in |
| Situation | From | To |
|---|---|---|
| Small drill bit | 0.125 in | 3.175 mm |
| Quarter inch | 0.25 in | 6.35 mm |
| Half inch | 0.5 in | 12.7 mm |
| One inch | 1 in | 25.4 mm |
| One foot | 12 in | 304.8 mm |
| One yard | 36 in | 914.4 mm |
Multiply by 25, then add 2%. 4 in is 100 + 2 = 102 mm, against an exact 101.6.
Drill bits, bolts and pipe sizes are often quoted in fractions of an inch. 3/8 in is 9.525 mm.
Convert fractions to decimals first: 5/16 in is 0.3125 in, which is 7.94 mm, not 5.16 mm.
An inch is exactly 25.4 mm, a figure fixed by the 1959 international agreement.
Related guides: Imperial vs metric: differences and quick conversion tricks