SI prefix

An SI prefix supplies an exact decimal factor when attached to a unit of measurement. For example, milli means $10^{-3}$ and micro means $10^{-6}$; a millimetre and a micrometre are correspondingly scaled metres. The prefix symbol and unit symbol form one unit that can itself be raised to a power.^[ref]

For an original conversion example,The area conversion squares the factor as well as the unit. Applying just one factor of $10^{-3}$ to that area would give $0.004{\mathrm{m}}^2$, a value 1000 times too large. Scientific notation makes the power explicit.

Prefix symbols are case-sensitive and cannot be stacked. For mass, prefixes attach to gram, because the base-unit name kilogram already contains a prefix. SI prefixes denote powers of ten, even in a computing context; they do not silently change into powers of two.^[ref] An exact unit conversion scales any stated measurement uncertainty with the value; it does not make an uncertain measurement exact.