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Don’t Add Tolerance by Rounding Off
(In accordance with Y14.5M-1994 standard)

PDF of this Tip

A common question relates to rounding off measurements or numbers when converting between metric and inch dimensions and tolerances. This Tip will address measurements. A subsequent Tip will take on conversions and rounding.

Y14.5 tells us in section 2.4 that all numbers are absolute. In other words, 1 means 1.000000…….forever. Y14.5 also refers us to ANSI/IEEE 268 standard on rounding off. In this standard we are told that you cannot increase the limits of a dimension as a result of rounding "where the original limits may not be violated." Let’s say the part thickness shown measures .236. Back in school most of us were told that we could round this to .24. That might be fine in sixth grade, but when measuring parts, it won’t pass. Bottom line, the .236 measurement is out of tolerance. Another consideration is the uncertainty of the measurement. Due to uncertainty of the measuring process, the .236 measurement may actually be less than the true value. Of course, the best approach is to stay away from the limits if your process is capable.

Back to Tips Tip added Aug 2005