How do I convert mm to inches?

Divide the millimeter value by 25.4. One inch is defined as exactly 25.4 millimeters, so 100 mm ÷ 25.4 = 3.937 inches. To go the other way, multiply inches by 25.4. The converter handles both directions — type in either field and the other updates instantly.

How many inches is 10 mm?

10 mm is about 0.3937 inches — just under 4/10 of an inch. As a quick sanity check: 1 mm is roughly 0.04 in, 25.4 mm is exactly 1 inch, and 50 mm is just under 2 inches. For any other value, type it into the converter.

What is 3 mm as a fraction of an inch?

3 mm is about 0.1181 inches, which is very close to 1/8 inch (0.125 in). Other handy near-matches: 6 mm ≈ 1/4 in, 10 mm ≈ 3/8 in (slightly over), 13 mm ≈ 1/2 in and 19 mm ≈ 3/4 in. These pairings are why metric and SAE wrench sets almost — but not quite — line up.

Is the 25.4 factor exact or rounded?

Exact. Since the international yard and pound agreement of 1959, the inch is defined as exactly 25.4 millimeters — a definition, not a measurement. Any rounding in results comes only from trimming long decimals for readability.

Is this converter free and private?

Yes. The math runs entirely in your browser with plain JavaScript — nothing you type is uploaded, stored or shared, there's no account and no usage limit. It also keeps working offline once the page has loaded.

Converters

MM to Inches Converter

Convert millimeters to inches (and back) instantly — with common values, the exact formula and a reference table.

Free forever
No sign-up
Runs in your browser

Share X LinkedIn

Common values

Millimeters

Inches

Type in either field — the other converts live. 1 inch = 25.4 mm exactly.

Result

—

Enter a value in Millimeters or Inches

How the conversion works

The inch is defined as exactly 25.4 millimeters, so the conversion is a single division or multiplication:

mm → inches: divide by 25.4 — 100 mm ÷ 25.4 = 3.937 in
inches → mm: multiply by 25.4 — 2 in × 25.4 = 50.8 mm

There is no rounding hidden in the factor itself: 25.4 has been the international definition since 1959, so results are as exact as the number of decimals you keep. Because millimeters are so small, mm-to-inch results often land on familiar fractions — 3 mm is almost exactly 1/8 inch, and 6 mm is a whisker under 1/4 inch.

Quick reference table

Millimeters	Inches
1 mm	0.0394 in
3 mm	0.1181 in (≈ 1/8 in)
5 mm	0.1969 in
10 mm	0.3937 in
20 mm	0.7874 in
25 mm	0.9843 in
25.4 mm	1 in exactly
50 mm	1.9685 in
100 mm	3.937 in
300 mm	11.811 in

How to use it

Type a value into either field — millimeters or inches.
The other side converts live; results are trimmed to four decimals.
Tap a common value chip (1, 5, 10, 25, 50, 100, 300 mm) to start from a typical measurement.
Copy puts the full conversion (e.g. 100 mm = 3.937 in) on your clipboard.

When you need this

Millimeters show up wherever precision matters: screw and bolt sizes, drill bits, jewelry (ring bands and gemstone diameters are always quoted in mm), watch case widths, TV and phone bezels, camera lens filters, and layer heights or nozzle sizes in 3D printing. If a spec sheet says the watch case is 42 mm and you think in inches, that's about 1.65 in — this tool answers exactly that kind of question. For larger metric lengths, the CM to Inches Converter works the same way; to break inch values down further, try the Inches to Feet Converter, and the general Unit Converter covers weight, volume, temperature and more in one place.

Private by design

The conversion runs in your browser — nothing is uploaded or tracked, and there's no sign-up. Browse more free utilities on the all tools page.

Frequently asked questions

Comet's got your back

Stuck on something? Every tool has a short guide and FAQ — and Comet can point you to the right spot.

Visit help centre

Related tools

All Converters tools →