= Millimetre
{title2=$1\,\mathrm{mm}=10^{-3}\,\mathrm{m}$}
= Millimeter
{synonym}
A <millimetre>, symbol $\mathrm{mm}$, is exactly one thousandth of a <metre>: $1\,\mathrm{mm}=10^{-3}\,\mathrm m$. The SI prefix \i[milli] specifies that exact scale factor; it does not state the uncertainty of a measurement expressed in the unit.<The International System of Units (SI), ninth edition, SI Brochure>[SI prefixes, Table 7]{ref}
For example, a thickness of $0.25\,\mathrm{mm}$ is $250\,\mathrm{\mu m}$ or $2.5\times10^{-4}\,\mathrm m$. Powers of a unit require powers of the conversion factor: $1\,\mathrm{mm}^2=10^{-6}\,\mathrm m^2$ and $1\,\mathrm{mm}^3=10^{-9}\,\mathrm m^3$. Thus a cube with 10 <millimetre> edges has volume $1000\,\mathrm{mm}^3=1\,\mathrm{cm}^3$, not $1000\,\mathrm{cm}^3$. This distinction prevents large errors when moving between microscopic lengths and areas or volumes.
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