How do I convert a millimeter tolerance to inches?

Last updated 10 August 2026

Divide both limits by 25.4 and round inward. 12.69 to 12.71 millimeters becomes .4997 / .5003 inches, and it needs four decimal places rather than three.

The arithmetic is the mirror of going the other way, but the precision question is not. A millimeter is about forty thousandths, so a band that reads comfortably at two decimals in millimeters needs four in inches to survive. Round this one to three and the inward rounding closes it to nothing.

The other half of the problem is what a metric print means by its decimals. A limit written 12.70 claims a different precision from one written 12.7, and converting the second as though it were the first invents tolerance the drawing never granted. Convert the limits, not the nominal, and if the drawing gives you a nominal and a symmetric tolerance, work out the limits first.

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Given	12.69 / 12.71 mm
Rounded to nearest, which is wrong	0.4996 / 0.5004 in
Rounded inward	0.4997 / 0.5003 in

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.
Rounding both ends to nearest, which is what a calculator does, would give 0.4996 / 0.5004 -- a band 0.00020 in wider than the drawing allows. A part inside that and outside this one would pass on the converted print and fail on the original.

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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How do I convert a millimeter tolerance to inches?

Worked

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