To convert millimeters to inches, divide by 25.4. 10 mm is 0.3937 in, a hair over 3/8 in, and 25 mm is 0.984 in, just under an inch.
Because US tools and hardware are sized in fractions, this converter also shows the nearest 1/16 inch, which is how you'd pick a drill bit, wrench or socket.
How to use the mm to inches converter
- Type the value in millimeters into the box. Decimals are fine.
- The result in inches updates as you type, along with the working.
- Need the other way? Switch the direction menu, or open the inches to mm converter.
Formula
One inch is exactly 25.4 millimeters by the 1959 international definition, so dividing by 25.4 is exact.
Worked example
Is a 19 mm socket the same as 3/4 in?
- 19 ÷ 25.4 = 0.7480 in.
- 3/4 in is 0.75 in (19.05 mm).
- Result: 0.748 in, about 0.05 mm smaller than 3/4 in. They're often treated as interchangeable, but the fit is slightly tighter.
Millimeters to inches chart
Common values, rounded to 4 decimal places.
| Millimeters (mm) | Inches (in) |
|---|---|
| 1 mm | 0.0394 in |
| 2 mm | 0.0787 in |
| 3 mm | 0.1181 in |
| 4 mm | 0.1575 in |
| 5 mm | 0.1969 in |
| 6 mm | 0.2362 in |
| 8 mm | 0.315 in |
| 10 mm | 0.3937 in |
| 12 mm | 0.4724 in |
| 13 mm | 0.5118 in |
| 15 mm | 0.5906 in |
| 16 mm | 0.6299 in |
| 19 mm | 0.748 in |
| 20 mm | 0.7874 in |
| 22 mm | 0.8661 in |
| 25 mm | 0.9843 in |
| 30 mm | 1.1811 in |
| 50 mm | 1.9685 in |
| 100 mm | 3.937 in |
Where millimeters show up
- Tools and fasteners: metric wrenches, sockets, bolts (M6, M8) and drill bits.
- Rainfall: most countries report rain in mm. 25 mm of rain is about an inch.
- Bikes and cameras: tire widths (25 mm, 28 mm) and lens focal lengths.
- Jewelry and hardware: ring widths, cable diameters, sheet thickness.
Quick mental math
Divide by 25, then take off about 1.6%. 50 mm ÷ 25 = 2, minus 0.03 = 1.97 in (exact: 1.9685). For fractions, remember 1/16 in ≈ 1.6 mm.
Related conversions
Going the other way: inches to mm 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
- The nearest-fraction value is rounded to 1/16 in; check tolerances before swapping metric and fractional parts.
- 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 inches is 1 mm?
About 0.03937 inches, roughly 1/25 of an inch.
Is 13 mm the same as 1/2 inch?
Close. 1/2 in is 12.7 mm, so a 13 mm wrench is slightly loose on a 1/2-inch nut.
How many mm is 1/16 inch?
1.5875 mm.
What is 10 mm in inches as a fraction?
0.3937 in, which is just over 3/8 in (0.375 in, or 9.525 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.