Calculating How Much Bacteriostatic Water to Add
To reach a chosen concentration, divide the vial amount by that concentration: diluent volume (mL) = vial amount (mg) ÷ target concentration (mg/mL).
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Working backwards from a target concentration
This is the inverse of the concentration formula and is the calculation most people actually need at the bench: you already know the concentration you want to read on the syringe, and you need to know how much liquid produces it.
Diluent volume (mL) = vial amount (mg) ÷ target concentration (mg/mL)
Examples
- 10 mg vial, target 5 mg/mL → 2 mL
- 10 mg vial, target 10 mg/mL → 1 mL
- 15 mg vial, target 5 mg/mL → 3 mL
- 30 mg vial, target 10 mg/mL → 3 mL
What bacteriostatic water is
Bacteriostatic water is sterile water containing roughly 0.9% benzyl alcohol as a preservative, which is why it is commonly chosen for multi-draw vials over plain sterile water. Its role in the calculation is purely as a volume — the arithmetic is identical for any diluent.
Check the vial before you draw
Confirm the target volume fits the vial's usable capacity, allowing for the space the lyophilised powder occupies and any headroom the stopper requires. If it does not fit, raise the target concentration and recalculate.