Inches to Millimeters (in to mm)

Formula: mm = in × 25.4

Inches to mm chart

Related: mm to inches · inches to cm · cm to mm · all conversions

Inches to Millimetres at a glance

Millimetres are the working unit in engineering drawings and manufacturing, where inch dimensions are usually converted before machining.

Both directions from one number

You only need to remember one factor, because the reverse is its reciprocal:

mm = in × 25.4
in = mm × 0.0394   (or divide by 25.4)

If a result looks about right but is off by a large multiple, you have almost certainly multiplied where you should have divided — checking the direction against the table above catches it immediately.

Rounding, and how precise to be

Converting does not create accuracy that was not in the original measurement. If someone tells you a distance in whole inches, the converted figure is only as good as that starting estimate — quoting it to four decimal places implies a precision nobody measured.

A practical rule: keep roughly the same number of significant figures you started with, and round only at the end. Rounding at each intermediate step compounds the error, which is how a chain of conversions drifts away from the right answer.

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