Convert millimetres to inches
Millimetres to inches for the workshop: decimal inches plus the nearest 1/64, 1/32 and 1/16 fraction, and a printable mm-to-fraction chart.
- Runs in your browser
- Nothing you enter is sent
- Free, no account
inches = millimetres ÷ 25.4 = 25.4 ÷ 25.4 = 1.0000 in 1 mm = 0.0393701 in
| millimetres | inches | Nearest 1/64 in |
|---|---|---|
| 1 mm | 0.0394 in | 3/64 in |
| 2 mm | 0.0787 in | 5/64 in |
| 3 mm | 0.1181 in | 1/8 in |
| 4 mm | 0.1575 in | 5/32 in |
| 5 mm | 0.1969 in | 13/64 in |
| 6 mm | 0.2362 in | 15/64 in |
| 8 mm | 0.3150 in | 5/16 in |
| 10 mm | 0.3937 in | 25/64 in |
| 12 mm | 0.4724 in | 15/32 in |
| 15 mm | 0.5906 in | 19/32 in |
| 16 mm | 0.6299 in | 5/8 in |
| 19 mm | 0.7480 in | 3/4 in |
| 20 mm | 0.7874 in | 25/32 in |
| 25 mm | 0.9843 in | 63/64 in |
| 25.4 mm | 1.0000 in | 1 in |
| 30 mm | 1.1811 in | 1 3/16 in |
| 40 mm | 1.5748 in | 1 37/64 in |
| 50 mm | 1.9685 in | 1 31/32 in |
| 100 mm | 3.9370 in | 3 15/16 in |
The shop-floor conversion
Millimetres to inches is the conversion you do standing at a bench with a metric drawing in one hand and an imperial tape in the other. It is one division: divide by 25.4. The inch is defined as exactly 25.4 mm, so 25.4 mm is exactly one inch, 12.7 mm is exactly half an inch, and 6.35 mm is exactly a quarter.
What makes this page different from a generic converter is that it does not stop at a decimal. A decimal inch is not something you can find on a rule. Three fraction columns are shown at once, at 1/64, 1/32 and 1/16, so you can pick the resolution your tooling can actually hold.
Metric and imperial drill and fastener sizes
Metric and imperial hardware are close enough to interchange and far enough apart to ruin a thread. The pairs that matter:
| Imperial | Exact mm | Nearest metric | Difference |
|---|---|---|---|
| 1/8 in | 3.175 mm | 3 mm | 0.175 mm undersize |
| 3/16 in | 4.7625 mm | 5 mm | 0.24 mm oversize |
| 1/4 in | 6.35 mm | 6.5 mm | 0.15 mm oversize |
| 5/16 in | 7.9375 mm | 8 mm | 0.06 mm oversize |
| 3/8 in | 9.525 mm | 9.5 mm | 0.025 mm undersize |
| 1/2 in | 12.7 mm | 13 mm | 0.3 mm oversize |
A 5/16 in bolt in an 8 mm hole is a clearance fit by 0.06 mm and perfectly fine. A 1/2 in bolt in a 13 mm hole rattles. Substituting the other way — a metric bolt in an imperial hole — is where things seize.
Sheet, wire and material thicknesses
Sheet metal and plywood are the classic trap, because the nominal name is not the measurement. "Half-inch" plywood is commonly 12 mm (0.472 in) rather than 12.7 mm, and 18 mm sheet is 0.709 in, not the 3/4 in (19.05 mm) it is sold against. Always convert the measured thickness, never the label.
For fine work, remember that one thousandth of an inch — a "thou" or "mil" — is 0.0254 mm. A 0.05 mm feeler gauge is about two thou.
How the fraction snapping works
The converter multiplies your millimetre value by the chosen denominator, rounds to the nearest whole number of those units, then reduces the fraction. 20 mm is 0.7874 in; at sixty-fourths that is 50.4 sixty-fourths, which rounds to 50/64 and reduces to 25/32. The result is labelled as approximate whenever the snap moved the value, so you always know whether you are looking at an exact figure or a practical one.
A printable reference
The table in the tool covers the values that come up daily, with decimal inches next to the fraction you will actually cut to. Print the page and it lands on one sheet — useful stuck inside a toolbox lid, and it works with the Wi-Fi off because the whole conversion lives in the page rather than on a server.
Questions
What is 1 mm in inches?
0.0393700787 inches — just over one sixty-fourth (1/64 in is 0.396875 mm, so 1 mm is a touch under 2.5 sixty-fourths).
Why show three different fractions?
Because different work has different resolution. A machinist works to 1/64 in, sheet goods and joinery to 1/32 in, and a builder's tape to 1/16 in. Showing all three lets you pick the one your tooling can actually hit.
Which metric drill size matches a 1/4 inch bit?
1/4 in is exactly 6.35 mm, so a 6.5 mm bit is the usual metric substitute and a 6 mm bit is slightly undersized. The chart on this page lists the common equivalents.
Is a millimetre the same as a "mil"?
No, and the confusion is expensive. A mil in US manufacturing is one thousandth of an inch — 0.0254 mm. A millimetre is roughly 39.4 mils.