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I have been asked to provide some of our technical docs in both metric and US units. For the most part, that is easy enough:

  • approx 1cm → approx 1/2"

  • 500μm → 0.02"

But then I start getting stuck:

  • 50μm → 0.002" or 2 thou or 2 mils? Is there a common symbol for thou/mills?
  • 20nm → ???

I imagine that people who care about nanometers have mostly gone metric now anyway, but it would still be good to provide US measurements for everything. What units are commonly used for lengths smaller than 0.001" in the US, and what symbols are used for them?

If it varies across different fields, the relevant ones would be optics, precision machining and mechanical engineering.

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    $\begingroup$ Using 1/2" instead of 1cm gives you about a 20% error. 1cm is a lot closer to (but a bit bigger than) 3/8". $\endgroup$
    – TimWescott
    Dec 4 '19 at 22:57
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    $\begingroup$ Remember than an inch is defined as 25.4mm. If you want to use "about" in technical documents you'd be better sticking to the convention that an inch is three barleycorns. $\endgroup$ Dec 21 '20 at 19:11
  • $\begingroup$ Don't duel with dual, as Pat Naughtin said. Providing measurements in both metric and US units only increases the time to switch over fully to the metric system and increases confusion in the meantime. The fact that you noticed there are no widely accepted US units around a micrometre or a nanometre should be a hint that the old way is broken and should not be continued. $\endgroup$
    – Nayuki
    Dec 22 '20 at 21:55
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    $\begingroup$ @Nayuki As a Brit with a science background, I agree that metric is the future and US Customary is daft. But if you're a small company pitching a product to a $100bn company, and they say they aren't metric-literate, it isn't a good time to fight that fight. $\endgroup$
    – Jack B
    Dec 23 '20 at 12:42
  • $\begingroup$ @MarkMorganLloyd Approx or about is fine in technical documents for things which are not specifications. e.g. "diameter: 18mm±0.05 (footnote) smaller packages could be manufactured with some loss in performance. The practical lower limit is approx. 1cm". The lower limit depends on the tolerable performance hit. saying 3/8" suggests that it's accurate to 1/8", when it really isn't. $\endgroup$
    – Jack B
    Dec 23 '20 at 12:53
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50μm → 0.002" or 2 thou or 2 mils?

I've seen both 0.002" and 2 mils. On a drawing it would always be 0.002". In a specification document it could be either. I've never seen 2 thou written in a formal specification (but I have heard people say it in the shop). But that might just be my experience. There could be variation from industry to industry.

Is there a common symbol for thou/mills?

I'm not aware of any. We just always wrote "mils" (not "mills")

What units are commonly used for lengths smaller than 0.001" in the US, and what symbols are used for them?

Colloquially, I know that people in precision machine shops refer to 0.1 mils (0.0001") as "tenths" (as in one ten thousandth of an inch). I've never seen that written in a formal specification document though, just as talking between machinists.

For US customary units, there's nothing smaller than an inch, you just start adding more zeros, applying prefixes, or using scientific notation. For example, it would be common to specify surface roughness in microinches.

20nm → ???

I would write it as "7.87e-7 inches"

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  • $\begingroup$ a mil is an angle. Ask Bob Snedeker (Sned at Cornell) $\endgroup$
    – D Duck
    Dec 7 '19 at 19:44
  • $\begingroup$ @D Duck, i guess it could be an angle in some context, but the OP is pretty clearly asking about lengths not angles. Mils are very common length units $\endgroup$
    – Daniel K
    Dec 7 '19 at 23:55
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    $\begingroup$ "mils" are rarely used in UK practice. I'd suggest avoiding, since there is too much risk of an incautious reader misinterpreting it as "millimetres". $\endgroup$ Dec 21 '20 at 19:09

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