Millimeters to inches

One millimeter is 0.0393701 inch. Enter any value and the answer updates as you type.

38 units in this converter

Value

Result

Decimals Decimals, fractions such as 3/4, and scientific notation such as 1.2e3 all work.

Millimeter to Inch at a glance

21 rows

Millimeter	Inch
1 mm	0.03937007874 in
2 mm	0.07874015748 in
3 mm	0.1181102362 in
4 mm	0.157480315 in
5 mm	0.1968503937 in
6 mm	0.2362204724 in
7 mm	0.2755905512 in
8 mm	0.3149606299 in
9 mm	0.3543307087 in
10 mm	0.3937007874 in
15 mm	0.5905511811 in
20 mm	0.7874015748 in
25 mm	0.9842519685 in
30 mm	1.181102362 in
40 mm	1.57480315 in
50 mm	1.968503937 in
75 mm	2.952755906 in
100 mm	3.937007874 in
250 mm	9.842519685 in
500 mm	19.68503937 in
1,000 mm	39.37007874 in

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Why people convert millimeters to inches

Millimeters and thousandths of an inch (mils) sound similar but are not close: 1 millimeter equals 0.0393701 inch, not 0.01 inch, a mix-up that matters in machining and fastener sizing, where an M6 bolt (6 millimeters across) is nowhere near a quarter-inch bolt despite both being small, everyday hardware sizes. Machinists and hobbyists working from a metric drawing on an imperial lathe or mill run this conversion constantly, since a tolerance given in hundredths of a millimeter needs converting precisely, not rounded, before it means anything on an imperial dial indicator.

Weather reporting is a less technical but very common source of this conversion: countries outside the US report rainfall and snowfall totals in millimeters, while US forecasts and news reports use inches, so a storm described as '50 millimeters of rain' needs converting to compare against a US flood-warning threshold stated in inches. The same gap shows up in tire tread depth, where many countries measure the legal minimum in millimeters while some US specification sheets still list it in thirty-seconds of an inch.

Popular length conversions

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How the millimeter to inch conversion works

The millimeter is one-thousandth of the meter, the SI base unit of length, while the inch is defined as exactly 25.4 millimeters.

Multiply a millimeter value by 0.03937007874 to get inches:

in = mm × 0.03937007874

One millimeter converts to 0.03937 inch. 25 millimeters, close to a US quarter's diameter, converts to 0.984252 inch — just under a full inch. And 500 millimeters, roughly a large monitor's width, converts to 19.685039 inches.

Millimeter measurements are often given to a tenth or hundredth of a millimeter in engineering contexts, and that precision carries straight through the conversion: a dimension given as 6.35 mm converts to exactly 0.25 inch, since machined parts are frequently designed around numbers that happen to land on clean fractions in the other system. For everyday measuring, three or four decimal places in the inch result is plenty.

The most common mistake is confusing millimeters with mils (thousandths of an inch) — two differently named small units that are not interchangeable. One millimeter is about 39.4 mils, not 1 mil, so swapping them in a spec sheet produces an error of nearly 40 times, which matters a great deal in precision machining.

Questions

How many millimeters are in one inch?

One inch equals 25.4 millimeters exactly.

Why does 10 millimeters convert to a decimal like 0.3937 inch instead of something rounder?

Because the inch, not the millimeter, carries the clean definition here (1 inch = 25.4 mm exactly); dividing in the other direction produces a repeating decimal, which is why 10 mm comes out to 0.3937007874 inch rather than a tidy fraction.

Is there a quick mental shortcut for millimeters to inches?

Divide by 25 instead of 25.4 for a rough estimate; it overstates the inch value by about 1.6%. For 100 millimeters, that gives 4 inches versus the precise 3.937.

What's the easiest way to avoid mixing up millimeters and mils?

Write out the unit name in full when the stakes are high — 'millimeter' versus 'mil' or 'thou' — since both can look similar in shorthand on a spec sheet, and a slipped decimal between them is a factor-of-40 error, not a rounding error.

Is the millimeter used here the same in every country?

Yes — the millimeter is part of the International System of Units (SI) and is defined identically everywhere it's used, so there's no regional variation to worry about the way there sometimes is with older, locally defined units.

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