How many millimeters are in a inch?

1 in = 25.4 mm. To convert any amount, multiply by 25.4.

How do you convert inches to millimeters?

Multiply the number of inches by 25.4 to get millimeters. The formula is shown in the "How it's calculated" section above.

Where does the conversion factor come from?

It's a published international standard. Most are defined exactly by international agreement (e.g. 1 inch = 25.4 mm exactly, 1 lb = 0.45359237 kg exactly). Source citations are listed under the formula above.

Unit converter — Length

Inches to millimeters converter

Convert inches (in) to millimeters (mm). Source-cited conversion factor, exact to the displayed precision.

Results are mathematically exact for the displayed precision. How we calculate.

How it's calculated

Convert inches to millimeters— the formula

Inches convert to millimeters by multiplying by the published conversion factor. The factor below is exact to the precision shown, sourced from the citation under the formula.

mm = in × 25.4

in = mm × 0.03937

SI base unit (BIPM). US customary lengths from the 1959 international yard agreement: 1 yd = 0.9144 m exactly, 1 in = 25.4 mm exactly, 1 mi = 1609.344 m exactly.

Questions

Frequently asked questions

How many millimeters are in a inch?
1 in = 25.4 mm. To convert any amount, multiply by 25.4.
How do you convert inches to millimeters?
Multiply the number of inches by 25.4 to get millimeters. The formula is shown in the "How it's calculated" section above.
Where does the conversion factor come from?
It's a published international standard. Most are defined exactly by international agreement (e.g. 1 inch = 25.4 mm exactly, 1 lb = 0.45359237 kg exactly). Source citations are listed under the formula above.