MM to inches

One inch is exactly 25.4 mm, so the divisor carries no rounding of its own.

Runs locally

ValueFromTo

You can type the unit too — 10 cm, 5'10", 3/4 in, 2 台尺

10 mm =

0.393701in

1 mm = 0.0393701 in
1 in = 25.4 mm
Formula× 0.0393701

Decimal places

One inch is exactly 2.54 cm. That is a definition agreed in 1959, not a measurement — so the conversion is exact in both directions.

Millimetres to inches, 1 to 100

One inch is exactly 25.4 mm, so millimetres divided by 25.4 gives inches with no rounding in the constant.

1 mm0.0393701 in
2 mm0.0787402 in
3 mm0.11811 in
4 mm0.15748 in
5 mm0.19685 in
6 mm0.23622 in
7 mm0.275591 in
8 mm0.314961 in
9 mm0.354331 in
10 mm0.393701 in
11 mm0.433071 in
12 mm0.472441 in
13 mm0.511811 in
14 mm0.551181 in
15 mm0.590551 in
16 mm0.629921 in
17 mm0.669291 in
18 mm0.708661 in
19 mm0.748031 in
20 mm0.787402 in
21 mm0.826772 in
22 mm0.866142 in
23 mm0.905512 in
24 mm0.944882 in
25 mm0.984252 in
26 mm1.02362 in
27 mm1.06299 in
28 mm1.10236 in
29 mm1.14173 in
30 mm1.1811 in
31 mm1.22047 in
32 mm1.25984 in
33 mm1.29921 in
34 mm1.33858 in
35 mm1.37795 in
36 mm1.41732 in
37 mm1.45669 in
38 mm1.49606 in
39 mm1.53543 in
40 mm1.5748 in
41 mm1.61417 in
42 mm1.65354 in
43 mm1.69291 in
44 mm1.73228 in
45 mm1.77165 in
46 mm1.81102 in
47 mm1.85039 in
48 mm1.88976 in
49 mm1.92913 in
50 mm1.9685 in
51 mm2.00787 in
52 mm2.04724 in
53 mm2.08661 in
54 mm2.12598 in
55 mm2.16535 in
56 mm2.20472 in
57 mm2.24409 in
58 mm2.28346 in
59 mm2.32283 in
60 mm2.3622 in
61 mm2.40157 in
62 mm2.44094 in
63 mm2.48031 in
64 mm2.51969 in
65 mm2.55906 in
66 mm2.59843 in
67 mm2.6378 in
68 mm2.67717 in
69 mm2.71654 in
70 mm2.75591 in
71 mm2.79528 in
72 mm2.83465 in
73 mm2.87402 in
74 mm2.91339 in
75 mm2.95276 in
76 mm2.99213 in
77 mm3.0315 in
78 mm3.07087 in
79 mm3.11024 in
80 mm3.14961 in
81 mm3.18898 in
82 mm3.22835 in
83 mm3.26772 in
84 mm3.30709 in
85 mm3.34646 in
86 mm3.38583 in
87 mm3.4252 in
88 mm3.46457 in
89 mm3.50394 in
90 mm3.54331 in
91 mm3.58268 in
92 mm3.62205 in
93 mm3.66142 in
94 mm3.70079 in
95 mm3.74016 in
96 mm3.77953 in
97 mm3.8189 in
98 mm3.85827 in
99 mm3.89764 in
100 mm3.93701 in

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The conversion above and the reference tables below are rendered on the server, so they work without JavaScript. Changing the value or the units needs JavaScript, because the calculation runs on your device rather than on ours.

Millimetres to inches is the conversion you need when a specification, a drill bit or a fastener is quoted in one system and your tools are in the other. One inch is defined as exactly 25.4 mm, so nothing is approximated except the display. Below is a complete 1 to 100 mm table, along with the two things that matter in workshop use: converting to sixteenths, and how many decimals are actually meaningful.

The conversion

Inches = millimetres ÷ 25.4. Millimetres = inches × 25.4. Because one inch is exactly 2.54 cm and there are ten millimetres in a centimetre, 25.4 is exact as well.

A useful anchor for shop work: 25 mm is 0.9843 in, so treating 25 mm as an inch is a 1.6 per cent error. On a single fastener that is a fraction of a millimetre and irrelevant; across a run of twenty holes it is 8 mm, which is a hole in the wrong place.

The reverse anchor: 1/16 inch is 1.5875 mm, and 1/32 inch is 0.79375 mm. Those two numbers cover most of the resolution a hand tool can deliver.

Millimetres to fractional inches, for drill bits and spanners

Imperial drill bits and spanners come in fractions, so a decimal answer needs one more step. Multiply the inch value by 16 and round: a 10 mm bolt is 0.3937 in, times 16 is 6.3, so the nearest imperial spanner is 6/16 = 3/8 inch at 0.375 in. That spanner is 0.019 in — half a millimetre — larger than the bolt head, which is loose enough to round the corners off a soft bolt.

This is why mixing metric and imperial spanners damages fasteners. The classic near-misses are 13 mm against 1/2 inch (0.512 vs 0.500), 19 mm against 3/4 inch (0.748 vs 0.750, which genuinely is interchangeable) and 10 mm against 3/8 inch (0.394 vs 0.375, which is not).

For drilling, the useful direction is usually imperial to metric: a 1/4 inch hole is 6.35 mm, so a 6 mm bit is undersize and a 6.5 mm bit is oversize. Neither is wrong, but you should know which one you are choosing.

Where millimetres and inches collide in real life

Photography is full of both. Lens focal lengths and filter threads are in millimetres worldwide, but sensor sizes are quoted in fractional inches that do not describe any physical dimension of the sensor at all — a "1/2.3 inch" sensor is about 6.2 mm wide. That naming comes from the outside diameter of the vidicon tubes it replaced, so no conversion applies. It is worth knowing before you try to convert it.

Watch straps are in millimetres, watch cases are usually in millimetres, but American watch parts are sometimes in inches. Bicycle components are split: wheels and tyres in inches for mountain bikes and in millimetres in the ISO system, seat posts and steerers in millimetres, some older frames in inches.

Plumbing and threads are the worst case, because the "size" is often nominal rather than measured. A 1/2 inch pipe does not measure half an inch anywhere. In Taiwan the trade calls the same thing 4分, which is 4/8 of an inch — the numbers only line up once you know that the 分 there means an eighth of an inch and not a metric unit.

How many decimals are actually meaningful

A machinist thinks in thousandths of an inch, and one thousandth is 0.0254 mm. So three decimal places in inches and two in millimetres describe about the same real precision. Four decimals in inches is tenths of a thousandth, which is grinding and metrology territory, not hand tools.

The default here is six significant figures, which is more than any of that needs, and it is chosen for a different reason: it is enough that a value converted out and back comes home to exactly the number you typed. If you switch to two decimal places you get a cleaner reading but lose that property, and the page says so when you switch.

Thermal expansion is worth remembering before chasing decimals. Steel grows about 12 micrometres per metre per degree Celsius, so a one metre steel part measured at 20 °C and again at 30 °C differs by 0.12 mm — bigger than most of the rounding people worry about.

Nothing is uploaded, and it works offline

The conversion is a division. It runs in your browser, the reference table below is rendered as static HTML on the server, and no request carries anything you type. You can verify that in the Network tab, or by disconnecting the network and reloading the page — it still works.

That matters less for a number than for a document, but the principle is the same one the whole site runs on: if the work can be done on your device, there is no reason to send anything anywhere.

Frequently asked questions

How many inches is 10 mm?

0.393701 inches, which is just over 3/8 inch (0.375). The exact value is 10 ÷ 25.4.

Is 25.4 mm per inch exact?

Yes. It follows directly from the 1959 definition of the inch as exactly 2.54 cm, so there is no rounding inside the constant.

What is the nearest imperial spanner to a 10 mm bolt?

3/8 inch at 0.375 in, against the bolt head at 0.3937 in. It is half a millimetre loose, which is enough to round the corners of a soft bolt, so use the metric one if you have it.

How do I convert millimetres to sixteenths of an inch?

Divide by 25.4 to get inches, then multiply the result by 16 and round. 12 mm is 0.4724 in, times 16 is 7.6, so about 8/16 — half an inch.

Can I convert a "1/2.3 inch" camera sensor size?

No, and neither can anything else. That figure is a legacy naming convention from vidicon tube diameters and does not describe a physical measurement, so no unit conversion applies to it.