To convert inches to millimeters, multiply by 25.4. 1/4 in is 6.35 mm, 1/2 in is 12.7 mm and 3/4 in is 19.05 mm.
The chart covers the fractional sizes printed on US tape measures, drill sets and plywood, which are the ones people most often need in metric.
How to use the inches to mm converter
- Type the value in inches into the box. Decimals are fine; type fractions as decimals (3/8 = 0.375).
- The result in millimeters updates as you type, along with the working.
- Need the other way? Switch the direction menu, or open the mm to inches converter.
Formula
One inch is exactly 25.4 millimeters by the 1959 international definition, so dividing by 25.4 is exact.
Worked example
Convert 5/16 in to millimeters.
- 5 ÷ 16 = 0.3125 in.
- 0.3125 × 25.4 = 7.9375.
- Result: 7.9375 mm, so an 8 mm bit is the closest metric size.
Inches to millimeters chart
Common values, rounded to 4 decimal places.
| Inches (in) | Millimeters (mm) |
|---|---|
| 1/16 in | 1.5875 mm |
| 1/8 in | 3.175 mm |
| 3/16 in | 4.7625 mm |
| 1/4 in | 6.35 mm |
| 5/16 in | 7.9375 mm |
| 3/8 in | 9.525 mm |
| 7/16 in | 11.1125 mm |
| 1/2 in | 12.7 mm |
| 5/8 in | 15.875 mm |
| 3/4 in | 19.05 mm |
| 7/8 in | 22.225 mm |
| 1 in | 25.4 mm |
| 1.25 in | 31.75 mm |
| 1.5 in | 38.1 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 4 in | 101.6 mm |
| 6 in | 152.4 mm |
| 12 in | 304.8 mm |
Fractional inches in practice
- Plywood and sheet goods: "3/4-inch" plywood is often actually 23/32 in (18.26 mm) thick.
- Drill bits and sockets: US sets step in 1/16 or 1/32 in.
- 3D printing and CAD: designs are usually in mm, so imperial parts need converting.
- Plumbing: pipe sizes like 1/2 in are nominal and don't match the actual outside diameter.
Quick mental math
Multiply by 25 and add a little (1.6%). For fractions, learn 1/16 in = 1.5875 mm and 1/8 in = 3.175 mm, then count up.
Related conversions
Going the other way: mm to inches converter. Related pages: inches to cm converter, cm to inches converter, feet to inches converter. For other units in the same family, use the full length converter.
Caveats and assumptions
- Results show up to 10 significant digits; the chart is rounded for reading. Keep full precision for engineering, medical or legal work.
Frequently asked questions
How many mm are in an inch?
Exactly 25.4 mm.
What is 1/4 inch in mm?
6.35 mm.
What is 3/4 inch in mm?
19.05 mm.
What is 1/8 inch in mm?
3.175 mm.
Sources
- NIST — SI Units: Length (1959 definition of the inch) — 1 in = 25.4 mm exactly
- NIST SP 811 Appendix B.9 — Conversion factors — inch to millimeter
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.