Bacteriostatic water — usually shortened to “BAC water” — is sterile water with a small amount of preservative added, typically 0.9% benzyl alcohol. That preservative is what lets a mixed vial be used more than once instead of requiring the entire vial to be used immediately, the way plain sterile water would.
How much to add
There’s no single universal amount — how much BAC water to add depends entirely on the vial’s labeled quantity and the concentration you’re aiming for. More water means a lower concentration and a larger liquid volume per dose; less water means a higher concentration and a smaller volume per dose. Many people default to a round number like 1 mL or 2 mL simply because it makes the resulting math easier to read on a syringe, not because it’s required.
Working out the amount that fits your syringes
If you already know how much water you want to add, our concentration calculator tells you the resulting concentration and syringe mark. If you’d rather work backward — you know your syringe sizes and want the calculator to suggest a water volume that fits them cleanly — switch to the “I Haven’t Mixed It Yet” tab, which is built for exactly that.
Storage and handling
How long a reconstituted vial remains usable, and how it should be stored, varies by compound and isn’t something a generic calculator can tell you — always follow the specific product’s own instructions.
Not a universal substitute
Bacteriostatic water isn’t appropriate for every compound. Some substances have their own specific diluent requirements. Never substitute a diluent unless the product’s own instructions specifically call for it.
This page is a research reference only and does not recommend an amount, schedule, or route of administration — see our Research & Medical Disclaimer.