Inches to mm converter
Imperial fixings, pipe sizes and sheet thicknesses show up on metric drawings all the time. One inch is exactly 25.4 mm, so the only real decision is rounding: keep two decimals on machined parts, round to the nearest millimetre on building drawings.
inches to mm converter
- Mm
- 254mm
- Centimetres
- 25.4cm
- Rounded to 1 mm
- 254mm
Formula
mm = inches × 25.41 inch = 25.4 mm exactly. The factor comes from the exact definition 1 in = 25.4 mm, so the conversion itself loses no precision — only rounding does.
Worked examples
| Input | Result |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 5 in | 127 mm |
inches to mm — common values
Exact values at 25.4 mm per in.
| Inches | Mm |
|---|---|
| 0.125 in | 3.175 mm |
| 0.25 in | 6.35 mm |
| 0.5 in | 12.7 mm |
| 0.75 in | 19.05 mm |
| 1 in | 25.4 mm |
| 1.5 in | 38.1 mm |
| 2 in | 50.8 mm |
| 2.5 in | 63.5 mm |
| 3 in | 76.2 mm |
| 4 in | 101.6 mm |
| 5 in | 127 mm |
| 6 in | 152.4 mm |
| 8 in | 203.2 mm |
| 10 in | 254 mm |
| 12 in | 304.8 mm |
| 16 in | 406.4 mm |
| 18 in | 457.2 mm |
| 20 in | 508 mm |
| 24 in | 609.6 mm |
| 30 in | 762 mm |
| 36 in | 914.4 mm |
| 48 in | 1,219.2 mm |
Questions
How many mm is 1 inch?
Exactly 25.4 mm. The value is defined, not measured, so it is precise at any number of decimals.
How do I convert fractional inches like 3/8" to mm?
Turn the fraction into a decimal first (3 ÷ 8 = 0.375), then multiply by 25.4: 0.375 × 25.4 = 9.525 mm.
Is a 1/2" pipe 12.7 mm?
Not usually. Nominal pipe sizes are trade names: a 1/2" pipe has an outside diameter of about 21.3 mm. Use the pipe schedule, not a unit conversion, for pipe sizes.
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