Convert inches to millimeters instantly. Enter a value below, or use the formula and reference table further down the page.
In practice
Inches to millimetres multiplies by 25.4, a figure that is exact because the inch is defined as 25.4 mm. It is the key conversion in machining, plumbing fittings, drill bits and phone and camera specs, where parts are often drawn in one unit and bought in the other. A quarter inch is 6.35 mm, a half inch 12.7 mm and a 3/4 inch 19.05 mm — so a 6 mm bit is slightly smaller than a 1/4 inch hole, which is why swapping sizes without checking can leave a loose fit.
How to convert inches to millimeters
Multiply the value by 25.4.
mm = in × 25.4Example: 10 in = 254 mm.
Inches to Millimeters conversion table
| Inches (in) | Millimeters (mm) |
|---|---|
| 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 |
Frequently asked questions
How many millimeters is 1 inch?
1 inch equals 25.4 millimeters.
What is the formula to convert in to mm?
To convert in to mm, multiply the value by 25.4. The full formula is: mm = in × 25.4.
How do I convert millimeters back to inches?
Use the same calculator above and type your value into the millimeters field instead — it converts in both directions. Or use the reverse formula: in = mm ÷ 25.4.
Is the in to mm conversion exact?
The conversion factor used here (25.4) is the standard internationally defined value. Results on this page are rounded to four decimal places for readability.