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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.

Jack B
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2 Answers2

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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"

Daniel K
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A microinch is one millionth (¹⁄₁₀₀₀₀₀₀) of an inch. Labeled as μin.

user54719
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