How to convert inches to millimeters
To convert inches to millimeters, multiply by 25.4. An inch is exactly 25.4 mm, a conversion used constantly in engineering, 3D printing, woodworking and hardware.
Inch to Millimeter conversion table
| Inch (in) | Millimeter (mm) |
|---|---|
| 0.1 in | 2.54 mm |
| 0.5 in | 12.7 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 |
| 20 in | 508 mm |
| 25 in | 635 mm |
| 50 in | 1,270 mm |
| 100 in | 2,540 mm |
| 500 in | 12,700 mm |
| 1,000 in | 25,400 mm |
About the units
Inch (in)
An inch is defined as exactly 2.54 centimeters. It is used in the US and UK for screen sizes, lumber and personal height.
Millimeter (mm)
A millimeter is one thousandth of a meter. It is the standard unit on engineering drawings, rulers and screw sizes.
How to use this converter
- Type the number of inches (in) in the input box.
- The result in millimeters appears instantly — no button needed.
- See the same value in every other length unit just below.
- Use “Swap units” to convert millimeters back to inches.
When you’ll use the inch to millimeter conversion
You are replacing a bolt measured at 3/8 inch and the hardware store sells metric sizes. Converting gives 9.525 mm, so a 10 mm bolt will not fit the same hole exactly.
A 3D-printing model uses inches but your slicer software expects millimeters. Converting the dimensions prevents a tiny print.
💡 For a quick estimate in your head, multiply by about 25 (the exact factor is 25.4).
Watch out for these mix-ups
- Assuming fractional inch sizes match metric sizes exactly.
- Using 25 instead of 25.4.
- Rounding precision measurements too early.
- Forgetting thread pitch differs between imperial and metric bolts.
Tips for better results
- Multiply inches by 25.4 for an exact answer.
- Keep at least two decimal places for engineering work.
- Check thread type as well as diameter for fasteners.
💡 Practical tip: Imperial and metric bolts with similar diameters usually have different threads and are not interchangeable.
Frequently asked questions
How many millimeters are in one inch?
One inch equals 25.4 millimeters. 25.4 is the exact factor used by this converter.
How do I convert inches to millimeters?
Multiply the value in inches by 25.4. To go the other way, multiply millimeters by 0.0393700787402.
What is 10 in in mm?
10 in = 254 mm. Also, 5 in = 127 mm and 100 in = 2,540 mm.
What is the difference between a inch and a millimeter?
An inch is defined as exactly 2.54 centimeters. It is used in the US and UK for screen sizes, lumber and personal height. A millimeter is one thousandth of a meter. It is the standard unit on engineering drawings, rulers and screw sizes.
Is this tool free and private?
Yes — the Inch to Millimeter is completely free with no sign-up, and it runs entirely in your browser, so the values you enter are never uploaded to our servers.
Related tools
📬 Get new tools first — free
Join Taurang to hear about new calculators, converters and features. No spam; unsubscribe anytime.