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Inches to millimeters (in to mm)

To convert inches to millimeters (in to mm): one inch is 25.4 millimeters, so 5 in is 127 mm.

Swap units: mm to in

In millimeters127 mm

Conversion factor Exact by definition. The inch has been exactly 2.54 cm and the pound exactly 0.45359237 kg since the 1959 international yard and pound agreement, so these are not measurements that can drift.
1 inch = 25.4 millimeters
The other way
1 millimeter = 0.0393700787 inches
Working
5 x 25.4 = 127
5 in in every unit
127 mm 12.7 cm 0.127 m 0.000127 km 5 in 0.41667 ft 0.13889 yd 0.000078914 mi 0.000068575 nmi
inches to millimeters
inmm
125.4
250.8
5127
10254
25635
501,270
1002,540

How the Unit Converter works

One inch is exactly 25.4 millimeters. That is a definition rather than a measurement: the imperial and US customary units were fixed against metric ones by the 1959 international yard and pound agreement, so this factor does not drift and will not be revised.

Worked through: 5 in x 25.4 = 127 mm.

Going the other way, one millimeter is 0.0393700787 inches.

To convert without a calculator, multiply by 25.4. Rounding that to 25 is within 1.6% for most purposes.

The direction a machinist or a 3D printer needs. CAD software and most manufacturing outside the US work in millimeters, so an imperial drawing has to be converted before anything is cut. Because 25.4 is exact, an inch dimension always lands on a terminating millimeter figure.

Precision is worth matching to the input. A measurement given as 1 in carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.

Unit Converter FAQ

How many millimeters are in a inch?

25.4. That is exact by definition: the 1959 international yard and pound agreement fixed it, and it does not change.

What is 5 in in millimeters?

127 mm. Multiply by 25.4: 5 in x 25.4 = 127 mm.

How do I convert millimeters back to inches?

Divide by 25.4, or equivalently multiply by 0.039370079. One millimeter is 0.039370079 inches. The two factors are reciprocals, so you only need to remember one of them.

Is there anything to watch out for with this conversion?

The direction a machinist or a 3D printer needs. CAD software and most manufacturing outside the US work in millimeters, so an imperial drawing has to be converted before anything is cut. Because 25.4 is exact, an inch dimension always lands on a terminating millimeter figure.

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