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Bacteriostatic water calculator
This is the reconstitution calculation reversed. Instead of adding water and seeing what concentration results, you state the concentration you want — or how much you want a single syringe unit to contain — and the calculator returns the diluent volume that produces it.
How it works
Two rearrangements of the same relationship:
- From a target concentration:
diluent volume = vial mass ÷ target concentration. - From a target per unit: a U-100 unit is 0.01 mL, so
target concentration = amount per unit ÷ 0.01, then the first formula applies.
Worked example, arithmetic only: a 5 mg vial where you want one U-100 unit to contain 25 mcg. That implies 2,500 mcg/mL, and 5,000 mcg ÷ 2,500 mcg/mL = 2 mL of diluent.
Practical constraints
- Vial capacity. A typical peptide vial holds a limited volume; a calculation returning 12 mL is arithmetically correct and physically useless.
- Readability. Very concentrated solutions mean tiny volumes that are hard to read on a barrel; very dilute ones mean large volumes and faster vial turnover.
- Consistency. Using the same target concentration across vials of the same compound makes your log directly comparable over time.
- Measurement accuracy. Measure the diluent, do not estimate it. Every downstream number depends on it.
Frequently asked questions
How much bacteriostatic water should I add to a 5 mg vial?
There is no single correct answer — it depends on the concentration you want to work with. Enter your vial mass and target concentration (or target amount per syringe unit) and this calculator returns the volume. Check the result fits the vial's physical capacity.
Does adding more water reduce the total amount of peptide?
No. The vial contains the same mass regardless. More water lowers the concentration, so each millilitre and each syringe unit contains less — but the total in the vial is unchanged.
Can I add more water to a vial later?
Doing so changes the concentration, which invalidates every earlier conversion of units to an amount for that vial. If you do it, close the old vial record and open a new one so your history stays interpretable.
Related pages
Stop recalculating this every time
PepCycle stores each vial once — mass, diluent volume, concentration — and converts automatically for every entry after that, in both directions, including entries you backdate.
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Last reviewed . All calculations run in your browser; no values are transmitted or stored.