Convert inches (fraction) to millimeters instantly. Enter a value below, or use the formula and reference table further down the page.
In practice
Fractional inches to millimetres is a daily conversion in woodworking, plumbing and metalwork, where tools and stock are marked in fractions like 1/8, 3/16 or 5/32 inch. Each fraction is converted by dividing the numerator by the denominator and multiplying by 25.4: a 1/8 inch is 3.175 mm, 1/4 inch is 6.35 mm and 3/8 inch is 9.525 mm. Notice that none of the common fractions land on whole millimetres, which is why metric and imperial bits, bolts and pipes are not interchangeable even when they look the same size.
How to convert inches (fraction) to millimeters
Multiply the value by 25.4.
mm = in × 25.4Example: 10 in = 254 mm.
Inches (fraction) to Millimeters conversion table
| Inches (fraction) (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 (fraction)?
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.