Inches to Millimeters Converter
Convert inches to millimeters instantly. 1 inch = 25.4 mm exactly. Formula, table and the rounding habit that causes real errors.
Written by Rahman · Last reviewed
Quick answer
One inch equals exactly 25.4 millimeters. Multiply inches by 25.4 to get millimeters, so a standard 0.25-inch drill bit is exactly 6.35 mm, not the "6 mm" it often gets rounded to.
| Metre (m) | 0.0254 |
| Kilometre (km) | 0.0000254 |
| Decimetre (dm) | 0.254 |
| Centimetre (cm) | 2.54 |
| Millimetre (mm) | 25.4 |
| Micrometre (µm) | 25400 |
| Nanometre (nm) | 25400000 |
| Mile (mi) | 0.00001578282828 |
| Yard (yd) | 0.02777777778 |
| Foot (ft) | 0.08333333333 |
| Inch (in) | 1 |
| Nautical mile (nmi) | 0.00001371490281 |
| Light year (ly) | 2.684782e-18 |
in to mm conversion table
| in | mm |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 4 in | 101.6 mm |
| 5 in | 127 mm |
| 6 in | 152.4 mm |
| 7 in | 177.8 mm |
| 8 in | 203.2 mm |
| 9 in | 228.6 mm |
| 10 in | 254 mm |
| 20 in | 508 mm |
| 30 in | 762 mm |
| 40 in | 1016 mm |
| 50 in | 1270 mm |
| 60 in | 1524 mm |
| 70 in | 1778 mm |
| 80 in | 2032 mm |
| 90 in | 2286 mm |
| 100 in | 2540 mm |
| 250 in | 6350 mm |
| 500 in | 12700 mm |
| 1000 in | 25400 mm |
How to convert in to mm
A common and costly mistake is rounding 25.4 down to 25 for quick mental maths: over a single inch the error is small, but over a 12-inch (304.8 mm) measurement it compounds to nearly 5 mm, enough to matter in carpentry, let alone machining, where tolerances are often measured in tenths of a millimetre. Woodworking plans published in the US often list every dimension only in inches and fractions of an inch, so working from one in a metric country means converting each measurement on the page before cutting anything.
On a standard ruler, the ⅛-inch markings between whole inches convert to messier metric values (3.175 mm, 6.35 mm, 9.525 mm…), which is one reason tradespeople who work across both systems keep a printed conversion table nearby rather than converting fractions on the fly.