= Micrometre
{title2=$\mu\mathrm{m}$}
A \b[micrometre], symbol $\mu\mathrm m$, is a <unit of measurement> for <length> equal to one millionth of a <metre>:
$$
\boxed{1\,\mu\mathrm m=10^{-6}\,\mathrm m=10^{-3}\,\mathrm{mm}=10^3\,\mathrm{nm}.}
$$
The <SI prefix> micro supplies the exact factor $10^{-6}$.<The International System of Units (SI), ninth edition, SI Brochure>[Chapter 3, Table 7: micro, symbol µ and factor; §2.3.1: metre]{ref}
For an invented measurement example, a metal foil of <thickness> $25\,\mu\mathrm m$ has <thickness> $0.025\,\mathrm{mm}$. Forty such layers have combined <thickness> $1000\,\mu\mathrm m=1\,\mathrm{mm}$ if they lie flat without gaps. The calculation makes a small <length> tangible without assuming a particular measuring instrument.
Unit conversion does not remove <measurement uncertainty>. A reported <thickness> of $(25\pm1)\,\mu\mathrm m$ becomes $(0.025\pm0.001)\,\mathrm{mm}$: both the value and its uncertainty receive the same exact scale factor.
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