Convert inches to millimeters
1 in = 25.4 mm · Formula and table below. Converting the other way? Millimeters to Inches →
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Other units
- Meter (m) 0.0254
- Kilometer (km) 0.0000254
- Decimeter (dm) 0.254
- Centimeter (cm) 2.54
- Millimeter (mm) 25.4
- Micrometer (µm) 25,400
- Nanometer (nm) 25,400,000
- Mile (mi) 0.000015783
- Yard (yd) 0.027777778
- Foot (ft) 0.083333333
- Inch (in) 1
- Nautical mile (nmi) 0.000013715
- Light year (ly) 2.684782×10⁻¹⁸
Reviewed on by the Skala Convert team
One inch is exactly 25.4 millimetres.
How many millimeters are in an inch?
mm = in × 25.4
This is the workshop conversion. Millimetres are the working unit of technical drawing, machining and European woodworking; inches are the working unit of American hardware. Anyone with a mixed toolbox converts between them constantly.
How do fractional inches convert to millimeters?
Inches in a workshop are fractional, not decimal. Drill bits, sockets and timber come as 1/16, 1/8, 3/16, 1/4 and so on. Converting them means turning the fraction into a decimal first: 3/8 inch is 0.375 in, which is 9.525 mm.
The near-misses cause real damage:
- 1/4 in = 6.35 mm, close to 6 mm
- 3/8 in = 9.525 mm, close to 10 mm
- 1/2 in = 12.7 mm, close to 13 mm
- 3/4 in = 19.05 mm, close to 19 mm — the closest pair
Why does a 1/2-inch pipe not measure half an inch?
Plumbing adds a trap: pipe sizes in inches are nominal, not actual. A "1/2 inch" pipe has neither an inner nor an outer diameter of 12.7 mm. The number is a legacy label, not a measurement, so converting it gives a figure that matches nothing physical.
Who defines the inch in millimeters?
The 1959 international yard and pound agreement, published in the US by the US National Bureau of Standards (now NIST), in an announcement effective 1 July 1959: one inch is exactly 25.4 mm. NIST Special Publication 811, the US guide to the SI repeats the figure as an exact conversion. On drawings, ISO 129-1 lets the drawing state one predominant unit and omit it from each dimension; in mechanical engineering that unit is almost always the millimetre, which is why so many imperial parts are specified in both.
Formula
Millimeters = Inches × 25.4
10 in = 254 mm
Conversion table
| in | mm |
|---|---|
| 1 | 25.4 |
| 2 | 50.8 |
| 5 | 127 |
| 10 | 254 |
| 20 | 508 |
| 25 | 635 |
| 50 | 1270 |
| 100 | 2540 |
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Frequently asked questions
What is 3/8 inch in mm?
9.525 mm. A 10 mm socket is close but not the same.
Is a 1/2 inch pipe actually 12.7 mm?
No. Pipe sizes in inches are nominal designations, not physical dimensions.
What is 25.4 mm in inches?
Exactly one inch. The relationship is a definition, not a measurement.