Millimetre ($1\mathrm{mm}=10^{-3}\mathrm{m}$)

A millimetre, symbol $\mathrm{mm}$, is exactly one thousandth of a metre: $1\mathrm{mm}=10^{-3}\mathrm{m}$. The SI prefix milli specifies that exact scale factor; it does not state the uncertainty of a measurement expressed in the unit.^[ref]

For example, a thickness of $0.25\mathrm{mm}$ is $250\mu \mathrm{m}$ or $2.5\times 10^{-4}\mathrm{m}$. 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.