Inches to Millimeters Converter

About the in to mm calculation

One inch is exactly 25.4 millimeters, so converting between them is a single multiplication. Enter any value above and the result updates as you type. Still standard for screen sizes, pipe diameters and tyre rims worldwide. The standard fine unit on engineering drawings and rainfall figures.

Formula

mm = in × 25.4

1 in = 25.4 mm
1 mm = 0.03937 in

Worked example

To convert 12 in, multiply by 25.4: 12 × 25.4 = 304.8 mm.

Frequently asked questions

How many millimeters are in a inch?

There are 25.4 millimeters in one inch. To convert any amount, multiply the number of inches by 25.4.

How do I convert millimeters back to inches?

Divide instead of multiplying, or equivalently multiply by 0.03937. One millimeter is 0.03937 inches.

Is this conversion exact?

Yes. Since the international agreement of 1959, the relationship between inches and millimeters is defined exactly rather than measured, so the only rounding here is in how many decimal places we show.

Where is each of these units used?

Inches: still standard for screen sizes, pipe diameters and tyre rims worldwide. Millimeters: the standard fine unit on engineering drawings and rainfall figures.

Sources

• NIST Special Publication 811 — Guide for the Use of the International System of Units