Reconstituting a lyophilized peptide is, mathematically, one of the simplest steps in a research workflow: you're combining a known mass of compound with a known volume of diluent to produce a solution of known concentration. This article covers that arithmetic on its own terms — as a record-keeping calculation, not as guidance on how a compound should be used.
The formula
Concentration is simply the mass of compound in the vial divided by the volume of diluent added:
Concentration (mg/mL) = Vial amount (mg) ÷ Diluent volume (mL)
That's the entire calculation. There's no rounding convention or hidden constant — the result is exact for whatever precision your inputs carry. What varies between labs is how the volume is measured and recorded, not the formula itself.
Worked examples
A 10 mg vial reconstituted with 2 mL of bacteriostatic water produces a solution of:
10 mg ÷ 2 mL = 5 mg/mL
The same 10 mg vial reconstituted with 5 mL instead produces a more dilute solution:
10 mg ÷ 5 mL = 2 mg/mL
And a 30 mg vial reconstituted with 3 mL produces a more concentrated one:
30 mg ÷ 3 mL = 10 mg/mL
Notice the relationship is inverse: for a fixed vial amount, more diluent always means a lower concentration, and less diluent always means a higher one. Keeping that relationship in mind is a useful sanity check before recording a figure — if you expected a higher concentration than what you calculated, double-check which volume you actually used.
Where precision matters
Two sources of error tend to creep into this calculation in practice. The first is unit confusion — mixing up milligrams and micrograms, or millilitres and microlitres, will throw the result off by three orders of magnitude. The second is measurement precision on the diluent volume itself: a syringe graduated in 0.1 mL increments carries more uncertainty than a calibrated pipette, and that uncertainty carries straight through to the final concentration figure. Recording the actual measured volume, not the intended one, keeps your batch notes accurate.
Using the calculator
Our Concentration Calculator performs exactly this division — select the vial amount, enter the diluent volume, and it returns the resulting concentration instantly, for your own record-keeping. It intentionally stops there: it does not calculate, recommend, or suggest doses or injection volumes, because that judgement sits outside the scope of a concentration tool and outside the scope of this site.
This article and the Concentration Calculator both perform concentration arithmetic only. Neither provides dosing recommendations or medical advice.