Bacteriostatic water (BAC water) is the solvent used to reconstitute lyophilized compounds. The volume you add does not change the total amount in the vial — it changes how concentrated the solution becomes. That concentration directly determines how large or small your draw will be.
How Concentration Is Set
Concentration is calculated by dividing the vial amount by the volume of BAC water added. A smaller volume produces a higher concentration. A larger volume produces a lower concentration.
5 mg vial + 1 mL BAC water → 5.0 mg/mL
5 mg vial + 2 mL BAC water → 2.5 mg/mL
5 mg vial + 5 mL BAC water → 1.0 mg/mL
The total amount in the vial stays at 5 mg in all three cases. Only the concentration — and therefore the draw volume — changes.
How Concentration Affects Draw Volume
Higher concentration means you draw a smaller volume to reach the same target amount. Lower concentration means a larger draw for the same target.
Target: 0.5 mg | Concentration: 5.0 mg/mL → Draw: 0.10 mL (10 units)
Target: 0.5 mg | Concentration: 2.5 mg/mL → Draw: 0.20 mL (20 units)
Target: 0.5 mg | Concentration: 1.0 mg/mL → Draw: 0.50 mL (50 units)
Choosing a Reconstitution Volume
There is no universally correct reconstitution volume. The practical goal is a draw volume large enough to measure accurately on your syringe — typically 10 units or more on a U-100 insulin syringe.
If your calculator result is coming back below 5 units, increase your BAC water volume. If your result is exceeding your syringe capacity, decrease it or switch to a larger syringe.
Stability Considerations
BAC water contains benzyl alcohol, which acts as a preservative to extend vial life after reconstitution. The volume you use does not affect this preservative function, but storing reconstituted vials correctly — refrigerated, away from light — is separate from the concentration calculation and applies regardless of how much BAC water was used.
Quick Reference
Concentration = Vial amount ÷ BAC water volume
More BAC water = lower concentration = larger draw
Less BAC water = higher concentration = smaller draw
Total vial amount does not change with reconstitution volume
Target draw range: 10–50 units on a U-100 syringe for best precision
Common Mistakes
More BAC water lowers concentration but not total amount. Your target amount stays the same — you just draw more liquid to get it.
The calculator uses the total reconstitution volume. Enter the final volume after all BAC water has been added.
Small draws are hard to measure accurately. Increase BAC water volume to produce a larger, more readable result.
This guide is for research-use calculator education only. It does not provide medical advice, treatment recommendations, or personalized dosing instructions.