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How do I convert an inch tolerance to millimeters?

Last updated 10 August 2026

Multiply both limits by 25.4, then round the lower up and the upper down. A half inch at plus or minus a thousandth becomes 12.675 to 12.725 millimeters.

The multiplication is the easy half and every calculator does it. The rounding is where converted drawings go wrong, because the intuitive rule and the correct rule differ, and the difference only ever shows up as a part that passed when it should not have.

Check your CAD system and your inspection software, because most of them round to nearest and do not say so. If a converted print is going to a customer or onto a traveler, the band it carries should be provably inside the one it came from.

Worked

Given	0.499 / 0.501 in
Rounding to nearest gives the same band	no difference at this precision
Rounded inward	12.675 / 12.725 mm

Rounded inward: the lower limit rounds up and the upper rounds down, so the converted band is contained inside the original. Anything that passes on this band would also have passed on the drawing it came from.

Open this case in the calculator, and change the numbers.

Computed by ctrlPlanner. ASME Y14.5, ISO 286.

This converts one value. ctrlPlanner holds both units on the same plan and applies the same inward rounding everywhere, so a whole print comes out in whichever unit the customer asked for.

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