Millimeters to Inches
Convert millimetres to inches and back, with fractional inch equivalents for hardware, timber and drill sizes where decimals are not what anyone actually uses.
One inch is exactly 25.4 millimeters — the definition, not an approximation.
| Millimeters | Inches |
|---|---|
| 1 | 0.04 |
| 5 | 0.2 |
| 10 | 0.39 |
| 25 | 0.98 |
| 50 | 1.97 |
| 100 | 3.94 |
| 150 | 5.91 |
| 200 | 7.87 |
| 250 | 9.84 |
| 300 | 11.81 |
The conversion is exact and the answer is unusable
The inch has been defined as exactly 25.4 millimetres since 1959, so there is no error in this conversion at all. Ten millimetres is 0.393701 inches and those digits are right.
They are also useless. A tape measure is not marked in thousandths of an inch — it is marked in halves, quarters, eighths and sixteenths. To use the number you have to pick the nearest mark, and picking the nearest mark is where the precision goes:
| Millimetres | Exact inches | Nearest 1/16 | Nearest 1/32 | Nearest 1/64 |
|---|---|---|---|---|
| 10 | 0.393701 | 6/16 (-0.475 mm) | 13/32 (+0.319 mm) | 25/64 (-0.078 mm) |
| 25 | 0.984252 | 16/16 (+0.400 mm) | 31/32 (-0.394 mm) | 63/64 (+0.003 mm) |
| 50 | 1.968504 | 31/16 (-0.788 mm) | 63/32 (+0.006 mm) | 126/64 (+0.006 mm) |
| 100 | 3.937008 | 63/16 (+0.012 mm) | 126/32 (+0.012 mm) | 252/64 (+0.012 mm) |
| 12.7 | 0.500000 | 8/16 (+0.000 mm) | 16/32 (+0.000 mm) | 32/64 (+0.000 mm) |
The last row is the exception that shows the rule: 12.7 mm is exactly half an inch, so it lands dead on a mark at every division. Every other row is an approximation dressed as a measurement.
How much a rule costs you, by division
Snapping to a mark can be wrong by at most half the spacing between marks. That bound is worth memorising, because it settles the question:
| Rule marked in | Worst error | Can hold a millimetre? |
|---|---|---|
| 8ths | 1.5875 mm | no |
| 16ths | 0.7937 mm | yes |
| 32ths | 0.3969 mm | yes |
| 64ths | 0.1984 mm | yes |
A rule marked in eighths cannot hold a millimetre — its worst error is larger than the unit you started with. Sixteenths can, but only just: three quarters of a millimetre of slop on every reading. If you measured something to the nearest millimetre and converted it for cutting on a sixteenths rule, you roughly doubled your uncertainty, and you did it in the step that looked like the exact one.
The rule that falls out of this: work in whichever system your tools are marked in, and convert once at the end if you have to — never in the middle of a job.
The two grids almost never meet
A sixteenth of an inch is 1.5875 millimetres — 127/80. For a whole number of millimetres to be a whole number of sixteenths, it has to be a multiple of 127. Between 1 and 300 millimetres there are exactly 2:
| Millimetres | Sixteenths | Inches |
|---|---|---|
| 127 | 80/16 | 5 |
| 254 | 160/16 | 10 |
Two values out of 300. Going the other way is no better: of the 768 sixty-fourths between 1/64 and 12 inches, only 2 land on a whole millimetre.
That is the whole difficulty in one line. The two systems agree at 25.4 mm and at multiples of it, and essentially nowhere else — so any conversion you do partway through a job lands between the marks on whichever rule you pick up next.
How to use
- Type a value in millimetres or inches.
- Read both the decimal inch and the nearest common fraction.
- Check the table for standard drill and fastener sizes.
- Watch for near-misses where a metric and an imperial size look interchangeable.
Frequently asked questions
How many millimetres are in an inch?
Exactly 25.4 mm, by definition. Because 1 inch is defined as 2.54 cm precisely, the millimetre conversion is exact too — a rare case where an imperial-to-metric conversion involves no approximation whatever.
What is 10 mm in inches?
0.3937 inches, which is just under 3/8 of an inch (0.375). The 0.0187-inch gap sounds trivial but is about half a millimetre, which is more than enough to round a bolt head or strip a screw.
Which fractional inch sizes are closest to common metric sizes?
The near-misses are worth knowing because they cause damage. 3 mm is close to 1/8 inch, 6 mm to 1/4, 10 mm to 3/8, 13 mm to 1/2, and 19 mm to 3/4. None are exact. The 1/2 inch and 13 mm pairing is the tightest, differing by only 0.2 mm, which is precisely why they are so often mixed up.
Why is timber sold in mixed units?
Because the trade converted in stages and never finished. Sheet goods are usually metric, framing lumber is often nominally imperial with metric actual dimensions, and fasteners can be either. A sheet described as 18 mm and one described as 3/4 inch differ by 1 mm, which shows up as a visible step where two sheets meet.
How precise is a millimetre in practice?
For woodwork, a millimetre is a visible gap. For machining, it is enormous — tolerances there are typically hundredths of a millimetre. For construction, it is usually below the accuracy of the measurement itself. What counts as precise depends entirely on what you are building.
How do I convert quickly in my head?
Divide by 25 rather than 25.4 and accept being about 1.6% high. 100 mm divided by 25 is 4 inches, against a true 3.94. For a rougher figure still, remember that 25 mm is about an inch and scale from there.
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