Free MM to Inches Converter
Convert millimeters to inches and back. One inch is exactly 25.4 mm, so the result is precise — type in either box for an instant answer.
Pick any units and edit either box — it converts instantly in your browser.
Quick answer
To convert millimeters to inches, divide the number of millimeters by 25.4, because one inch is exactly 25.4 mm. For example, 100 mm ÷ 25.4 = 3.94 inches. To go back, multiply inches by 25.4.
Formula & method
inches = millimeters ÷ 25.4
- millimeters — length in mm
- 25.4 — millimeters in one inch (exact)
Inverse: millimeters = inches × 25.4.
Common millimeters to inches
| Millimeters (mm) | Inches (in) |
|---|---|
| 1 mm | 0.039 in |
| 5 mm | 0.197 in |
| 10 mm | 0.394 in |
| 25.4 mm | 1.000 in |
| 50 mm | 1.969 in |
| 100 mm | 3.937 in |
| 300 mm | 11.811 in |
Examples
- Input
- 25.4 mm
- Result
- 1 inch
- Why
- 25.4 ÷ 25.4 = 1, since an inch is defined as 25.4 mm.
- Input
- 10 mm
- Result
- 0.3937 in
- Why
- 10 ÷ 25.4 = 0.3937.
- Input
- 100 mm
- Result
- 3.937 in
- Why
- 100 ÷ 25.4 = 3.937.
When to use this tool
- Reading engineering, woodworking, or 3D-printing dimensions in the other unit.
- Converting screw, bolt, or part sizes.
- Following plans that mix metric and imperial.
Common mistakes
- Confusing millimeters with centimeters; 10 mm = 1 cm, so 1 inch is 25.4 mm but 2.54 cm.
- Using 25 instead of the exact 25.4.
- Reading the decimal of inches as a fraction directly — 0.5 in is a half inch, but 0.3937 in is not 39/100 of anything tidy.
Frequently asked questions
How do I convert mm to inches?
Divide the millimeters by 25.4. The result is exact because an inch is defined as exactly 25.4 mm.
How many millimeters are in an inch?
Exactly 25.4 millimeters.
What’s the difference between mm and cm to inches?
There are 10 mm in a centimeter. So an inch is 25.4 mm or 2.54 cm — divide mm by 25.4, or cm by 2.54.
How do I convert inches back to millimeters?
Multiply inches by 25.4, or just type a value into the inches box here.
Is this accurate enough for engineering?
Yes. The 25.4 factor is exact, and the tool shows several decimals; round to the precision your work requires.
What is a common metric bolt size in inches?
For example, an M6 (6 mm) bolt is about 0.24 inches in diameter. An M10 is about 0.39 inches. Imperial bolts use fractional inch sizes (1/4", 3/8", etc.).
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- ✓ No sign-up required
- ✓ Runs entirely in your browser — nothing is uploaded.
- ✓ Formula and method shown above
Provided “as is” for general information only — results may be inaccurate, so verify before you rely on them. No warranty; use at your own risk.
Built and reviewed by HIFreeTools against the formula shown above and any authoritative references cited on this page. See our methodology and editorial standards.
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