Micrometre ($\mu \mathrm{m}$)

A micrometre, symbol $\mu \mathrm{m}$, is a unit of measurement for length equal to one millionth of a metre:The SI prefix micro supplies the exact factor $10^{-6}$.^[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 $(25\pm 1)\mu \mathrm{m}$ becomes $(0.025\pm 0.001)\mathrm{mm}$: both the value and its uncertainty receive the same exact scale factor.