Convert inches to millimetres
Convert fractional or decimal inches to millimetres exactly — 3/8, 1 1/4 and 0.375 all work — with a full sixteenths-to-mm reference table.
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millimetres = inches × 25.4 = 0.375 × 25.4 = 9.525 mm 1 in = 25.4 mm
| inches | millimetres | Input as a fraction |
|---|---|---|
| 0.0625 in | 1.588 mm | 1/16 in |
| 0.125 in | 3.175 mm | 1/8 in |
| 0.1875 in | 4.763 mm | 3/16 in |
| 0.25 in | 6.350 mm | 1/4 in |
| 0.3125 in | 7.938 mm | 5/16 in |
| 0.375 in | 9.525 mm | 3/8 in |
| 0.5 in | 12.700 mm | 1/2 in |
| 0.625 in | 15.875 mm | 5/8 in |
| 0.75 in | 19.050 mm | 3/4 in |
| 0.875 in | 22.225 mm | 7/8 in |
| 1 in | 25.400 mm | 1 in |
| 1.5 in | 38.100 mm | 1 1/2 in |
| 2 in | 50.800 mm | 2 in |
| 3 in | 76.200 mm | 3 in |
| 4 in | 101.600 mm | 4 in |
| 6 in | 152.400 mm | 6 in |
| 12 in | 304.800 mm | 12 in |
One multiplication, no rounding
Inches to millimetres is × 25.4, and because that factor is a definition rather than a measurement, the answer is exact. 1 in is 25.4 mm. 1/16 in is 1.5875 mm. 0.001 in — one thou — is 0.0254 mm. Nothing in that chain is approximate, which is why imperial-to-metric drawing conversion is safe to do once and trust.
The sixteenths ladder
Most imperial dimensions arrive as fractions, so here is the ladder every workshop ends up memorising half of:
| Fraction | Decimal in | Millimetres |
|---|---|---|
| 1/16 | 0.0625 | 1.5875 |
| 1/8 | 0.125 | 3.175 |
| 3/16 | 0.1875 | 4.7625 |
| 1/4 | 0.25 | 6.35 |
| 5/16 | 0.3125 | 7.9375 |
| 3/8 | 0.375 | 9.525 |
| 7/16 | 0.4375 | 11.1125 |
| 1/2 | 0.5 | 12.7 |
| 5/8 | 0.625 | 15.875 |
| 3/4 | 0.75 | 19.05 |
| 7/8 | 0.875 | 22.225 |
| 1 | 1.0 | 25.4 |
Notice that every one of those millimetre values ends in a 5 or a 0 — an artefact of 25.4 having only one decimal place. If your converted figure has a long tail of digits, you either started from a decimal inch or something upstream was already rounded.
Type the fraction, not the decimal
The input accepts 3/8, 1 1/4, 1¼ and 0.375 interchangeably. Converting the fraction to a decimal in your head first is an unnecessary step and a place to drop a digit — particularly with thirds and sixty-fourths, where the decimal is long and the fraction is short.
Nominal sizes are not measurements
A "1/2 inch" pipe is not half an inch across anything. Nominal pipe size is a trade designation inherited from wall thicknesses that stopped being standard a century ago: NPS 1/2 pipe has an outside diameter of 0.840 in (21.34 mm). The same applies to lumber, where a 2×4 finishes at 1.5 × 3.5 in (38 × 89 mm), and to screw gauges, which are an index rather than a length.
The rule that saves you: convert the number your calliper reports, never the number on the label.
Tolerances survive the conversion
A tolerance converts the same way the dimension does. ±0.005 in becomes ±0.127 mm. What does not survive is the implied precision of a rounded figure: writing 12.7 mm from 1/2 in implies ±0.05 mm, which is a far tighter claim than the drawing made. Keep the tolerance explicit and the number of decimals honest.
Questions
What is 1 inch in mm?
Exactly 25.4 mm. Multiply by 25.4 and you are done — there is no rounding involved at any stage.
Can I type 3/8 rather than 0.375?
Yes. Fractions, mixed numbers and unicode fractions are all parsed, which matters because imperial drawings and hardware are labelled in fractions, not decimals.
What thread size matches a 1/2 inch pipe?
A nominal 1/2 in pipe is not 12.7 mm across anything — nominal pipe sizes are trade names, not measurements. Convert the measured outside diameter instead, and check against the standard your fittings use.
Why are my two converted numbers 0.1 mm apart?
Almost always because one of them was rounded to two decimal places somewhere upstream. Increase the precision selector here and you will see where the difference entered.