Millimetres to Inches Converter (mm to in)

Convert millimetres to inches instantly. 1 mm = 0.0394 in. Includes the formula, a full conversion table and a calculator that works offline.

The formula

To convert millimetres to inches, multiply by 0.0394:

in = mm × 0.0394

Going the other way, divide instead — or multiply by 25.4, which is the same thing:

mm = in × 25.4

Millimetres to Inches conversion table

Millimetres	Inches
1 mm	0.0394 in
2 mm	0.0787 in
3 mm	0.1181 in
4 mm	0.1575 in
5 mm	0.1969 in
6 mm	0.2362 in
7 mm	0.2756 in
8 mm	0.315 in
9 mm	0.3543 in
10 mm	0.3937 in
15 mm	0.5906 in
20 mm	0.7874 in
25 mm	0.9843 in
30 mm	1.181 in
40 mm	1.575 in
50 mm	1.969 in
75 mm	2.953 in
100 mm	3.937 in
250 mm	9.843 in
500 mm	19.685 in
1000 mm	39.37 in

Inches to Millimetres

Inches	Millimetres
1 in	25.4 mm
2 in	50.8 mm
3 in	76.2 mm
4 in	101.6 mm
5 in	127 mm
6 in	152.4 mm
7 in	177.8 mm
8 in	203.2 mm
9 in	228.6 mm
10 in	254 mm

What that actually looks like

A conversion factor is hard to judge on its own. These are quantities most people already have a feel for, converted for you:

• A credit card is 0.76 mm thick — 0.0299 in
• A standard pencil lead is 0.5 mm — 0.0197 in
• A typical phone is 8 mm thick — 0.315 in

Why both units are still in use

Almost every country measures length in metres and centimetres. The United States is the significant exception, and the United Kingdom is genuinely split: road signs are in miles, a person's height is in feet and inches, but a bottle of water is in millilitres and a running race is in kilometres. That is why this conversion comes up so often — not because one system is replacing the other, but because both are in daily use side by side.

Mistakes worth avoiding

The most common mistake is mixing feet and inches in one number. Five foot ten is 5.833 feet, not 5.10 — the inches are twelfths, not hundredths. Convert the inches separately, or work in inches throughout and divide at the end.

The other habit worth breaking is rounding too early. Convert first, round once at the end, and you avoid the drift that turns a small approximation into a visible error by the third calculation.

Doing it in your head

When you only need a rough answer, divide by 25 — which lands within 1.6 percent of the true value. Ten millimetres works out at roughly 0.4 inches that way, against 0.3937 exactly. An estimate is enough for deciding whether something will fit or whether a figure sounds plausible. When the number has to be right — because you are buying, cooking or building to it — use the calculator and let it carry the decimals.

Common questions

How many inches are in one millimetre?

One millimetre equals 0.0394 inches. The calculator above handles any value, including decimals and very large numbers, and updates as you type.

Is this an exact conversion?

The factor is accurate to the precision shown, which is far beyond what any practical measurement requires. The table rounds for readability; the calculator does not.

Does this work offline?

Yes. The conversion runs in your browser with no network request, so it works on a plane, in a basement, or on a phone with no signal once the page has loaded.