= Microlitre
{title2=$\mathrm{\mu L}$}
A \b[microlitre], symbol $\mathrm{\mu L}$, is one millionth of a <litre>, a unit of <volume>. Since a <litre> is exactly one cubic <decimetre>,
$$
1\,\mathrm{\mu L}=10^{-6}\,\mathrm{L}=10^{-3}\,\mathrm{mL}=10^{-9}\,\mathrm{m^3}=1\,\mathrm{mm^3}.
$$
The prefix micro fixes the factor $10^{-6}$; it does not describe how accurately an instrument can deliver that <volume>.<The International System of Units (SI), ninth edition, SI Brochure>[Version 2.01, chapter 3 Table 7 and chapter 4 Table 8 note d: SI prefixes and the exact litre–cubic-decimetre relation.]{ref}
A $20\,\mathrm{\mu L}$ sample occupies $0.020\,\mathrm{mL}$, the same <volume> as twenty cubes each one <millimetre> on a side. Delivering $19\,\mathrm{\mu L}$ 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.<Gilson Guide to Pipetting>[Second edition, pp. 46–49: gravimetric pipette verification and conversion from measured mass to volume.]{ref}
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