inches to mm formula
To convert inches to millimeters, multiply the number of inches by 25.4. In symbols: mm = in × 25.4.
One inch equals 25.4 millimeters, and one millimeter equals 0.0393700787402 inches. The inch is defined as exactly 25.4 mm, so every result here is exact.
Worked example
Suppose you have 4 inches and need the answer in millimeters.
- Write the formula: mm = in × 25.4.
- Put in the number: 4 × 25.4 = 101.6.
- Round to suit the job. 101.6 mm is accurate enough for most everyday uses.
So 4 in is 101.6 mm. To check, convert back: 101.6 mm is 4 in.
Common inches to millimeters values
| inches (in) | millimeters (mm) |
|---|---|
| 1 | 25.4 |
| 2 | 50.8 |
| 5 | 127 |
| 10 | 254 |
| 20 | 508 |
| 50 | 1,270 |
| 100 | 2,540 |
Reference points
Some familiar amounts in inches, converted to millimeters:
- 0.25 in, a quarter inch bolt: 6.35 mm
- 1 in, one inch: 25.4 mm
- 3 in, a typical phone width: 76.2 mm
- 36 in, a yard: 914.4 mm
Where you will use it
Machinists, engineers and 3D printing hobbyists switch between inch drawings and millimeter parts. Screw sizes, pipe diameters and tool sizes are often listed both ways.
Quick mental math
Multiply by 25. Add 1.6 percent if you need a closer match; 4 in is exactly 101.6 mm. Rule of thumb: multiply by 25. For 4 in this gives 100 mm against the exact 101.6, an error of 1.6 percent.
Going the other way
Need the reverse? Use the mm to inches converter. For comparison, 1 mm is 0.039370079 in. To compare every length unit at once, open the length converter.
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