Inches to MM Converter
Turn a fractional or decimal inch from a US drawing, drill chart or caliper into millimetres so a metric print or CNC job can use the same length.
Updated
How to use
- Enter inches as a decimal. 1/4 is 0.25, 3/8 is 0.375, 5/16 is 0.3125.
- Read millimetres. One inch is 25.4 mm; half an inch is 12.7 mm.
- Keep extra decimals if you will machine or 3D-print the part.
- If the source is already in millimetres, use the reverse page instead of multiplying twice.
How it works
US drawings and hardware catalogs still mix fractional inches with metric production. This page multiplies inches by 25.4 so a shop that programs in millimetres does not have to keep a paper chart on the wall. A quarter inch is 6.35 mm. A half inch is 12.7 mm. Three quarters is 19.05 mm. Those three numbers appear on almost every imperial drill and wrench set.
Hex keys and sockets are a frequent mismatch. A 5/32 inch key is 3.9688 mm — close to 4 mm, not the same. A 3/16 inch bolt head is 4.7625 mm. If a manual says “use a 5 mm hex” and you only have fractional keys, convert the millimetre size the other way; this page is for the inch → mm direction.
Sheet metal and 3D printing use millimetres even when the designer thinks in inches. A 0.125 in wall (1/8) is 3.175 mm. Rounding that to 3 mm changes the part. Enter the full decimal and leave the extra digits until you choose a manufacturing tolerance.
This is ten times the centimetre conversion (inches × 2.54). Use millimetres when the destination drawing’s unit is mm. Use centimetres when you are marking a sewing pattern or a school ruler that only has cm ticks.
Pipe “inch” names are not a conversion. 1/2 inch NPS is not 12.7 mm OD. Convert the measured dimension from the spec sheet, not the trade name.
Formula
millimetres = inches × 25.4
Worked examples
One inch on a drawing
1 in → 25.4 mm
A half-inch stock size
0.5 in → 12.7 mm
Eighth-inch sheet
0.125 in → 3.175 mm
Common inch fractions in millimetres
| in | mm |
|---|---|
| 0.25 | 6.35 |
| 0.5 | 12.7 |
| 1 | 25.4 |
| 2 | 50.8 |
| 4 | 101.6 |
| 6 | 152.4 |
Useful notes
- Fractions must be decimals: 1/8 = 0.125, 1/16 = 0.0625, 1/32 = 0.03125.
- Nominal pipe and lumber names are not measured inches.
- Inches × 2.54 gives centimetres; this page reports millimetres.
FAQ
- How many millimetres are in an inch?
Exactly 25.4 millimetres.
- How many millimetres is 1/8 inch?
0.125 × 25.4 = 3.175 mm.
- How many millimetres is 3/8 inch?
0.375 × 25.4 = 9.525 mm.
- Is this different from inches to centimetres?
Only by a factor of ten. Inches × 2.54 gives centimetres; inches × 25.4 gives millimetres.
- Why doesn’t 1/2 inch pipe become 12.7 mm?
Pipe inch sizes are trade names. Convert the actual outside diameter from the specification, not the name.
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