A microlitre, symbol , is one millionth of a litre, a unit of volume. Since a litre is exactly one cubic decimetre,The prefix micro fixes the factor ; it does not describe how accurately an instrument can deliver that volume.[ref]
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.
A sample occupies , the same volume as twenty cubes each one millimetre on a side. Delivering 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 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]
Gilson Guide to Pipetting. Second edition, pp. 46–49: gravimetric pipette verification and conversion from measured mass to volume.
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