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How many decimal places should a converted tolerance have?

Last updated 10 August 2026

The test is not aesthetic. Convert, round inward, and check that the result is contained in the original band. If rounding inward at your chosen precision leaves nothing, or inverts the pair, you need another decimal place, and the tight case above is what that looks like.

Carrying more decimals than the process can hold is its own mistake in the other direction: a limit stated to five decimals of a millimeter claims a precision no shop-floor gauge will resolve, and invites an argument at inspection.

Worked

• 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 12.697 / 12.703 -- a band 0.0020 mm 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.

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