Last updated 10 August 2026
Because both limits were rounded to nearest. Round lower limits up and upper limits down instead, and the converted band stays inside the original.
Rounding a single measurement to nearest is correct. Rounding a pair of limits to nearest is not, because a limit pair is not two numbers, it is a statement about what will be accepted. Round each end to nearest and about half the time both ends move outward, producing a band that admits parts the original drawing did not.
The worked case above is a real one. The honest conversion and the naive one differ by a thousandth of a millimeter, which sounds like nothing until you notice it is entirely in the direction that passes a part the drawing rejects, on a document somebody is going to inspect against.
Round inward instead: lowers up, uppers down. The cost is that you can occasionally reject a part that was in tolerance by a fraction of the rounding increment, which is the direction an inspection error is allowed to go.
| Given | 0.50003 / 0.50203 in |
|---|---|
| Rounded to nearest, which is wrong | 12.701 / 12.752 mm |
| Rounded inward | 12.701 / 12.751 mm |
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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