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We have standard profiles such as this type of tube with outer diameter of 60.3mm, used for example in railing. Why is it 60.3mm rather than exactly 60mm? In what application does the extra 0.3mm make a significant difference? I've also seen profiles ending in .7mm. These sound inconvenient to produce compared to full millimiters.

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    $\begingroup$ Lines up close with 2 3/8", granted that's a weird imperial size as well. Could be grandfathered in from older standards. $\endgroup$
    – jko
    Commented Jun 4, 2021 at 15:13
  • $\begingroup$ Don't know if that's the reason, but machined parts will often have "round" dimensions and buying 60,3 mm raw material will be more economical than taking the next one, say 70 mm, and turning 10 mm off. $\endgroup$
    – OpticalResonator
    Commented Jun 4, 2021 at 18:57
  • $\begingroup$ IMHO, its mostly a type of technical debt (or inertia). I think the sizes were first determined for imperial and then while at some point people started to work more with mm, it made more sense in terms of compatibility to round to the closest decimal of a mm. . $\endgroup$
    – NMech
    Commented Jun 4, 2021 at 19:03
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    $\begingroup$ Structurally it doe not make a significant difference, but it can be for the purpose of fitting the parts. $\endgroup$
    – r13
    Commented Jun 4, 2021 at 19:10
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    $\begingroup$ I bet it is 2 3/8in, that's 60.325mm. I disagree that it's an unusual diameter, 1/8 inch increments are about the smallest that might matter to an architect. $\endgroup$
    – Drew ♦
    Commented Jun 6, 2021 at 2:12

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Circular hollow sections were standardised a long time ago and in inches for historical reasons. This should be obvious if you look at a complete list and convert the units.

The question is better asked the other way around: Why should we change the standard diameter from 60.3mm to 60.0mm, which would an additional cost for the manufactorers, when the 0.3mm difference doesn't make a significant difference anyway?

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  • $\begingroup$ And then add all the tools in all the machine shops that would need changing, and also all the fittings, clamps and boxes that would need changing. Starts to be a good reason things continue... $\endgroup$
    – Solar Mike
    Commented Jun 6, 2021 at 6:34
  • $\begingroup$ Google shows that 60.3mm is about 2.374 inches. That sort of repeats my question. Why isn't it 2.4 inches? In what application does the 0.03 inches make much difference? $\endgroup$
    – S. Rotos
    Commented Jun 6, 2021 at 6:58
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    $\begingroup$ Inches are typically/historically divided into fractions, not decimals - 1/2, 1/4, 1/8, 1/16 etc. 0.375 is 3/8, which is a common inch fraction. $\endgroup$
    – Jonathan R Swift
    Commented Jun 6, 2021 at 8:50

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