1 inch to mm — 25.40 millimetres
1 inch is exactly 25.40 millimetres. Multiply by 25.4 to convert any value from inches to mm.
Inches to mm
LiveThe formula
Take the number of inches and multiply it by 25.4: 1 × 25.4 = 25.40 mm. The factor comes from the international definition of the units, so the result is exact rather than an estimate — only the displayed value is rounded.
| Result | 25.40 mm |
|---|---|
| In millimetres | 25.4 mm |
| In metres | 0.0254 m |
| In feet | 0.083 ft |
Where this measurement turns up
- Pipe diameters
- bolt sizes
- drill bits
- plumbing fittings
Worth knowing
Millimetres dominate engineering drawings because whole numbers are easier to tolerance. That is why an imperial pipe labelled 1 inch appears as 25.4 mm on a technical drawing, and why mixing the two systems on one drawing is considered bad practice.
The mistake to avoid
Confusing millimetres with centimetres is the most common error here, since both are metric and both start with the same digits at a glance. Double-check the unit label on a drawing before cutting material, not just the number.
Questions people ask
1 inch is exactly 25.40 millimetres. The calculation is 1 × 25.4.
Multiply the value in inches by 25.4. To go the other way, divide the mm figure by the same number.
The conversion factor is defined by international standard, so the underlying result is exact. We round the display to a sensible number of decimal places, which is why you may see a slightly shorter figure than a calculator gives.
For a rough estimate, multiply by 25 — close enough for everyday use. For anything that needs to be right, such as a form, a cut list or a shipping label, multiply by exactly 25.4 instead, since the shortcut drifts further from the true value as the number grows.
Need the reasoning, not just the number?
Our guides explain how each measurement is defined, where it is used and which mistakes cost people money.