1 inch equals 25.4 millimeters. Enter a value below to convert instantly.
Converting inches to millimetres is standard practice whenever a US drawing is manufactured on metric equipment — which is most of the world's machine shops. Millimetres are also the honest unit for small tolerances: a thousandth of an inch is 0.0254 mm, and writing it that way makes the precision obvious.
Need the other direction? Convert millimeter to inch →
Multiply the number of inches by 25.4 to get millimeters.
| Inches (in) | Millimeters (mm) |
|---|---|
| 1 | 25.4 |
| 2 | 50.8 |
| 3 | 76.2 |
| 5 | 127 |
| 10 | 254 |
| 20 | 508 |
| 50 | 1,270 |
| 100 | 2,540 |
| 500 | 12,700 |
| 1,000 | 25,400 |
Metric bolts are specified by shank diameter in millimetres (M6, M8, M10) while US hardware is fractional-inch (1/4, 5/16, 3/8). They are close enough to start threading and different enough to strip — an M8 is 8 mm, a 5/16 inch bolt is 7.94 mm, and neither belongs in the other's hole.
CAD files circulate in both systems. A part drawn in inches and sliced in millimetres will print at the right size only if the conversion happened at import; a silent unit mismatch shows up as a part exactly 25.4 times too big or too small.
Tap drill charts are published in both systems, and the nearest metric bit to a fractional one is often within a few hundredths of a millimetre. Converting first tells you whether 'close enough' really is — for a clearance hole it usually is, for a tapped hole it usually is not.
Tyre widths, sheet metal gauges and rainfall totals are all quoted in millimetres internationally. A 225 mm tyre and an 8.86-inch tyre are the same tyre, but only one of those numbers appears on the sidewall.
Values people actually look up, with the reason each one comes up. Every figure is calculated with the same engine as the converter above.
| Inch (in) | Millimeter (mm) | What it is |
|---|---|---|
| 0.001 | 0.0254 | One thou — the machinist's tolerance unit |
| 0.125 | 3.175 | One eighth of an inch |
| 0.25 | 6.35 | A quarter inch |
| 0.5 | 12.7 | Half an inch |
| 1 | 25.4 | The base conversion |
| 2 | 50.8 | A common pipe nominal size |
| 8.5 | 215.9 | The short side of US Letter paper |
| 11 | 279.4 | The long side of US Letter paper |
A millimetre is one thousandth of a metre. An inch is exactly 25.4 millimetres, which follows directly from the exact 2.54 cm definition. In practice millimetres and inches are used at different scales: millimetres for anything you would machine or fasten, inches for anything you would buy off a US shelf. The unit that carries the tighter tolerance in a drawing is usually the one the part was designed in.
One thousandth of an inch is 0.0254 mm, so a millimetre value quoted to two decimal places is finer than a typical shop tolerance and three decimals is almost always false precision. The reverse trap is worse: converting a whole-millimetre dimension into inches produces long decimals (7 mm becomes 0.2755905… in), and copying all of those digits into a drawing implies a tolerance nobody can hold. Round to the precision the part actually needs, and state the tolerance separately.
A quarter inch is 6.35 mm against M6's 6 mm. The bolt will enter and it will not hold thread properly. The same trap catches M8 against 5/16 inch and M10 against 3/8 inch.
A model that arrives 25.4 times too large or too small is almost never corrupt — it is an inch file read as millimetres or the reverse. Check one known dimension before you scale anything.
That is a 1.6% error, which over a 300 mm run is nearly 5 mm. Fine for a rough sketch, disastrous for a hole pattern.
Content on this page last reviewed 12 August 2026. Spotted an error? Report it — see also how these values are calculated.
1 in expressed in every other length unit on this site.
The inch (in) is a Imperial unit of length. Defined internationally as exactly 25.4 millimeters since 1959.
The millimeter (mm) is a SI unit of length.
A thou (0.001 inch) is 0.0254 mm, or 25.4 micrometres. Machinists often work to a few thou, which is a few hundredths of a millimetre.
Divide the fraction out first, then multiply by 25.4. Three eighths of an inch is 0.375 in, and 0.375 × 25.4 = 9.525 mm. Common fractions land on tidy millimetre values: 1/2 in is exactly 12.7 mm and 1/4 in is exactly 6.35 mm.
No — nominal pipe sizes are labels, not measurements. A 1/2 inch NPS pipe has an outside diameter of about 21.3 mm. Convert the stated outside diameter, never the nominal size, when the fit matters.
One inch equals 25.4 millimeters.
Multiply the number of inches by 25.4 to get millimeters.
One millimeter equals 0.039370079 inches.
This conversion is common when measuring height, screen sizes, travel distances, fabric, and construction dimensions.