MM to Inches Converter

Convert millimeters to inches instantly, with a reference table for common hardware and screen sizes.

Millimeters

Inches

1 mm = 0.0394 in

Multiply millimeters by 0.03937008 to get inches.

The link carries the values you entered, so whoever opens it sees the same result. Nothing is stored on our side — the numbers travel in the link itself.

Millimeters to inches reference table

The conversions people look up most often, rounded to 4 decimal places.

Common millimeters to inches conversions
Millimeter (mm)	Inch (in)
1 mm	0.0394 in
2 mm	0.0787 in
3 mm	0.1181 in
5 mm	0.1969 in
10 mm	0.3937 in
12 mm	0.4724 in
15 mm	0.5906 in
20 mm	0.7874 in
25 mm	0.9843 in
50 mm	1.9685 in
100 mm	3.937 in
200 mm	7.874 in
300 mm	11.811 in

The millimeters people look up most

6 mm = 0.2362 in: 6 mm plywood, sold as quarter-inch. It is 0.236 in - thinner than the imperial sheet it replaced, which shows when it has to sit flush.
10 mm = 0.3937 in: A shade over 3/8 in. Close enough to swap drill bits in softwood, nowhere near close enough in steel.
16 mm = 0.6299 in: A common watch lug width. Straps are sold in even millimetres only, so measure the gap before ordering.
20 mm = 0.7874 in: The other standard strap width, 0.79 in. Watch listings almost never print the imperial figure.
25.4 mm = 1 in: Exactly one inch. The inch was redefined as 25.4 mm by international agreement in 1959, so this is a definition rather than a measurement.
50 mm = 1.9685 in: Just under two inches. Useful for timber, where a "2 by 4" is neither 2 nor 4 inches once it has been planed.
100 mm = 3.937 in: Close enough to four inches to mislead, and far enough off to matter in a tight fit.

How the mm to inches calculation works

There are exactly 25.4 millimeters in an inch, so:

inches = millimeters ÷ 25.4

Millimeters are the working unit for anything precise — hardware, machining, phone screens, sheet material — which is why this conversion tends to matter more at the fine end than the coarse one. Getting four decimal places right is often the point.

A worked example: a 10 mm bolt is 10 ÷ 25.4 = 0.3937 inches. That is very close to, but not the same as, the 3/8" (0.375") imperial size — a 5.5% difference, and enough that the two are not interchangeable in a threaded hole. This is the classic reason metric and imperial hardware should not be mixed.

Two things worth keeping in mind:

Fractional inches need a second step. Shops usually want sixteenths or thirty-seconds, not decimals. Multiply the decimal by 16: 0.3937 × 16 = 6.3, so 10 mm is roughly 6/16" (3/8"), slightly over.
A millimeter is about 1/25th of an inch. Dividing by 25 instead of 25.4 gets you within 1.6% — good enough to eyeball a drill bit, not good enough to cut to.

Reverse the conversion by multiplying inches by 25.4, or tap the swap control on the converter above.

Frequently asked questions

Can I use a 13 mm spanner on a half-inch bolt?

It will go on, and that is the problem. A half-inch head measures 12.7 mm, so a 13 mm spanner leaves 0.3 mm of slack spread across the flats - enough to round the corners off as soon as the bolt is tight. A half-inch spanner on a 13 mm nut simply will not go on, which is the safer of the two failures. Keep the two sets separate.

Why is plywood sold as 18 mm when the shop calls it three-quarter inch?

They are different sheets. Three quarters of an inch is 19.05 mm, and the metric sheet that replaced it is 18 mm - just over a millimetre thinner. In a rebate, a housing joint or a stack of six shelves that difference is visible and cumulative. Measure the sheet you have rather than trusting the name on the rack, because both are sold under the same description.

What is a thou, and how does it relate to millimetres?

A thou is one thousandth of an inch, which is 0.0254 mm - near enough 25 microns. Ten thou is a quarter of a millimetre and a hundred thou is 2.54 mm. Machinists in both systems still talk in thou because it is a convenient size for a tolerance, so a part specified within five thou is being asked to hold about 0.13 mm.

MM to Inches Converter