Exact inch and millimeter conversion
Inches to MM Converter
Enter decimal or fractional inches and convert with the exact 25.4 mm-per-inch factor. The calculation stays in your browser.
How to convert inches to mm
The international inch is defined as exactly 25.4 millimeters. Multiply inches by 25.4; divide millimeters by 25.4 for the reverse direction.
millimeters = inches × 25.4
inches = millimeters ÷ 25.4
1 international inch = 25.4 millimeters exactly
Example: 3 1/2 in = 3.5 × 25.4 = 88.9 mm, which is also 8.89 cm or 0.0889 m.
When inch-to-mm conversion is useful
Fractional inches are common on U.S. drawings and hardware, while metric specifications usually use millimeters.
Machining and fabrication
Convert an inch drawing dimension before entering a millimeter value in CAD, CNC, laser-cutting, or 3D-printing software.
Fasteners and material thickness
Compare fractional-inch stock or nominal hardware sizes with a metric catalog, then confirm tolerances and thread specifications separately.
Screens, frames, and products
Translate an inch measurement into millimeters, centimeters, and meters without repeating the calculation for each metric scale.
Common inches to mm chart
These values use exactly 25.4 millimeters per inch. Decimal displays retain four places even when trailing zeros are significant to the selected format.
| Inches (in) | Millimeters (mm) |
|---|---|
| 0.0625 | 1.5875 |
| 0.125 | 3.1750 |
| 0.25 | 6.3500 |
| 0.375 | 9.5250 |
| 0.5 | 12.7000 |
| 0.75 | 19.0500 |
| 1 | 25.4000 |
| 1.5 | 38.1000 |
| 2 | 50.8000 |
| 3 | 76.2000 |
| 4 | 101.6000 |
| 6 | 152.4000 |
| 8 | 203.2000 |
| 10 | 254.0000 |
| 12 | 304.8000 |
| 24 | 609.6000 |
Fractions, exact conversion, and displayed precision
Enter a simple fraction such as 3/8 or a mixed number such as 1 1/2. The converter evaluates that inch amount before applying the 25.4 factor. A pasted inch mark, in, inch, inches, or mm also selects the matching direction.
The 25.4 conversion factor is exact. The decimal-place menu controls only the displayed result, so a shorter display can be rounded even though the converter keeps the unrounded value for the calculation.
Centimeters and meters are derived from the same inch input, using exactly 2.54 cm and 0.0254 m per inch. They are included to make product and drawing comparisons easier, not as separate measurements.
A conversion does not add measurement accuracy. For fits, machining tolerances, threads, or safety-critical parts, preserve the source measurement's precision and follow the drawing or manufacturer specification.
Sources and method
The unit constants come from the primary standards below. ToolNeko applies those constants in a browser-only calculation, then rounds only the displayed result.
- NIST — SI Units: Length States that the standard inch equals exactly 25.4 millimeters.
- NIST Guide to the SI, Appendix B — Conversion Factors Provides the U.S. customary-to-SI length conversion factors and guidance for presenting converted values.
Frequently asked questions
How many millimeters are in one inch?
One international inch equals exactly 25.4 millimeters. The factor is defined, rather than measured or rounded.
How do I convert inches to mm?
Multiply the number of inches by 25.4. For example, 2 inches × 25.4 = 50.8 millimeters.
What is 1/2 inch in millimeters?
One-half inch is exactly 12.7 millimeters because 0.5 × 25.4 = 12.7.
Can I enter fractional inches?
Yes. Enter simple fractions such as 3/8, mixed numbers such as 1 1/2, or forms with an inch suffix such as 3/8 in or 3/8".
How do I convert mm back to inches?
Divide millimeters by 25.4, or enter the value in the millimeter field. For example, 88.9 mm ÷ 25.4 = 3.5 inches.