Reconstitution concentration calculator

Concentration arithmetic for reconstituted lyophilized material. This calculator converts a stated mass and a diluent volume into a concentration, and back again. It is unit arithmetic — the same calculation used for any reagent prepared from a solid.

Output is given in millilitres. We do not convert to insulin-syringe units. Concentration arithmetic is laboratory maths that applies to any reagent; a conversion into syringe gradations has exactly one application, and it is not a laboratory one.

The arithmetic

Concentration is mass divided by volume. Ten milligrams of solid taken up in two millilitres of diluent gives five milligrams per millilitre, which is 5,000 micrograms per millilitre. The volume containing a given quantity is that quantity divided by the concentration.

Two things this arithmetic assumes, both of which are frequently untrue. First, that the vial contains the mass printed on the label — see reading a certificate of analysis, because purity and content are different measurements. Second, that the solid has fully dissolved. A cake that has gelled or only partially gone into solution produces a concentration that is neither uniform nor calculable. Our lab guides cover why that happens.

Volume displacement

The solid occupies volume once dissolved. For milligram quantities in millilitre volumes the displacement is small enough to fall inside normal pipetting error, and the calculation above ignores it. At higher solid-to-solvent ratios it stops being negligible, and final volume should be measured rather than assumed.