inches to mm
1 inch equals 25.4 millimeters. Formula: millimeters = inches × 25.4.
= 12.7 mm
Other direction: mm to inches
Formula
millimeters = inches × 25.4
Example: 0.5 in = 12.7 mm.
When you need inches to mm
Screws, sockets, plumbing fittings, and lumber from US suppliers are labeled in inches — often as fractions. Multiplying by 25.4 turns those into millimeters for metric calipers and drawings: 3/8 inch is 9.525 mm, and 1/2 inch is 12.7 mm.
The chart below includes the common fractional sizes (1/8, 1/4, 3/8, 1/2, 3/4) alongside whole inches, because those fractions are what you actually see printed on tool walls and packaging.
Common in to mm values
| Inches (in) | Millimeters (mm) |
|---|---|
| 0.125 | 3.175 |
| 0.25 | 6.35 |
| 0.375 | 9.525 |
| 0.5 | 12.7 |
| 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 |
FAQ
How many mm are in an inch?
There are exactly 25.4 millimeters in one inch. The definition is exact, so inch-to-mm conversions have no rounding error.
What is 3/8 inch in mm?
3/8 inch = 0.375 × 25.4 = 9.525 mm. A 10 mm socket is slightly larger.