Unit converter
Inches to Millimeters Converter
Use this inches to millimetres converter when imperial dimensions have to fit metric parts. It suits workshop drawings, imported fasteners, camera accessories, jewellery blanks, bike components, and 3D-print tolerances. The inch has a toBase value of 0.0254 metres, while the millimetre has a toBase value of 0.001 metres, so the exact factor is 25.4. Enter an inch value, including decimals from fractional sizes, and the page returns the matching millimetre length without changing the underlying precision.
1 inch = 25.4 millimeters
Common inches to millimeters values
| Inches | Millimeters |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 5 in | 127 mm |
| 10 in | 254 mm |
| 20 in | 508 mm |
| 50 in | 1,270 mm |
| 100 in | 2,540 mm |
How to convert
- Write the inch measurement as a decimal, so 7/8 inch becomes 0.875 inch.
- Use the factor 25.4, calculated from 0.0254 / 0.001 in the unit table.
- Multiply the inch value by 25.4 to get millimetres.
- Keep the full result until the final rounding step if the part has a tolerance.
- Mental trick: call each inch 25 mm, then add 0.4 mm per inch for a closer check.
millimetres = inches * 25.4
The conversion engine first expresses inches in metres. Since 1 inch is 0.0254 metres and 1 millimetre is 0.001 metres, 0.0254 / 0.001 gives the exact multiplier 25.4.
- in
- length entered in inches
- 25.4
- exact millimetres per inch
Worked example
A 2.75 inch drawer pull is 2.75 * 25.4, which equals 69.85 mm. That makes it clear whether a 70 mm centre-to-centre fitting will line up with the existing holes.
Key facts
- 1 in is exactly 25.4 mm.
- 0.5 in is exactly 12.7 mm.
- 0.125 in is exactly 3.175 mm.
- 100 mm is about 3.937007874 inches.
Tips
- Convert fractional inch sizes to decimals before multiplying.
- For workshop notes, two decimal places are usually clearer than long raw output.
- When ordering parts, check whether the listed size is nominal or measured.
Frequently asked questions
How many millimetres are in 1 inch?+
There are exactly 25.4 millimetres in 1 inch, using the toBase ratio 0.0254 / 0.001.
What is 3/8 inch in millimetres?+
3/8 inch is 0.375 inch. Multiplying 0.375 by 25.4 gives 9.525 mm.
Can I use this for thread and pipe labels?+
Use care. Thread, pipe, and trade sizes often name a standard rather than the measured outside diameter.
How many inches are in 1 millimetre?+
One millimetre is 1 / 25.4 inches, which is about 0.03937007874015748 inches.
Things to watch
- Rounding small imperial parts to whole millimetres can change fit and clearance.
- Do not treat trade pipe sizes as simple inch measurements.
Sources
Last updated: 2026-01-01
Conversions use internationally defined factors. Provided for general use; verify critical measurements independently.