Microlitre ($\mu \mathrm{L}$)

A microlitre, symbol $\mu \mathrm{L}$, is one millionth of a litre, a unit of volume. Since a litre is exactly one cubic decimetre,The prefix micro fixes the factor $10^{-6}$; it does not describe how accurately an instrument can deliver that volume.^[ref]

A $20\mu \mathrm{L}$ sample occupies $0.020\mathrm{mL}$, the same volume as twenty cubes each one millimetre on a side. Delivering $19\mu \mathrm{L}$ instead produces a 5% shortfall: a small absolute volume error can therefore matter greatly in a small-scale chemical reaction.

The microlitre is a volume unit, not a mass unit. To infer delivered volume from a balance measurement, use $V=m/\rho$ with the appropriate liquid density; careful calibration also accounts for effects such as air buoyancy and evaporation. Thus “one microlitre equals one milligram” is an approximation for water under particular conditions, not a unit identity. Gilson's calibration protocol illustrates repeated gravimetric measurements that separate systematic delivery error from scatter between deliveries.^[ref]