Inches to Millimeters Converter
There are exactly 25.4 millimeters in one inch. To convert inches to millimeters, multiply by 25.4.
How to Convert Inches to Millimeters
To convert inches to millimeters, multiply by 25.4.
For example: 5 inches × 25.4 = 127 millimeters.
Where This Comes Up
Machining, 3D printing, bike components and hardware tolerances are all specified in millimeters, while American stock material and drill bits are sized in inches and inch fractions. This is the conversion that lets the two meet on one drawing.
A 3/8-inch bolt is 9.525 millimeters across, which is why a 9 mm wrench is too small and a 10 mm wrench is sloppy on it.
What People Get Wrong
Rounding a quarter inch to 6 mm instead of 6.35 mm loses 0.35 mm, which is more than most press-fit tolerances allow. Keep two decimals on machining dimensions.
Inches to Millimeters Conversion Table
| Inches (in) | Millimeters (mm) |
|---|---|
| 1 | 25.4 |
| 2 | 50.8 |
| 3 | 76.2 |
| 4 | 101.6 |
| 5 | 127 |
| 6 | 152.4 |
| 7 | 177.8 |
| 8 | 203.2 |
| 9 | 228.6 |
| 10 | 254 |
| 11 | 279.4 |
| 12 | 304.8 |
| 13 | 330.2 |
| 14 | 355.6 |
| 15 | 381 |
| 16 | 406.4 |
| 17 | 431.8 |
| 18 | 457.2 |
| 19 | 482.6 |
| 20 | 508 |
| 25 | 635 |
| 30 | 762 |
| 40 | 1,016 |
| 50 | 1,270 |
| 75 | 1,905 |
| 100 | 2,540 |
| 250 | 6,350 |
| 500 | 12,700 |
| 1,000 | 25,400 |
About These Units
The inch
The inch is exactly 2.54 centimeters by international agreement. Americans use it for screen sizes, lumber, rainfall, tire rims and body height together with feet.
The millimeter
A millimeter is one thousandth of a meter. It is the unit of choice for engineering drawings, rainfall and anything small enough that centimeters would need decimals.
Definitions follow NIST Special Publication 811, Appendix B. See how we calculate for the exact values and rounding rules.
Frequently Asked Questions
How many millimeters is 1 inch?
One inch is exactly 25.4 millimeters.
What is 1/16 of an inch in millimeters?
One sixteenth of an inch is 1.5875 millimeters.
How do I convert inch fractions to millimeters?
Divide the fraction out first, then multiply by 25.4. For 5/8 that is 0.625 times 25.4, which is 15.875 millimeters.
Is a 10 mm bolt the same as 3/8 inch?
No. 3/8 inch is 9.525 mm, so the two are about half a millimeter apart — enough that the threads will not match.