Bacteriostatic Water Calculator
The amount of bacteriostatic water you add doesn't change how much peptide is in the vial — it sets the concentration, which decides how big each dose's draw is. This calculator shows you both.
The formula
Concentration = peptide amount ÷ water added
So a vial reconstituted with less water is more concentrated (smaller draws), and more water is more dilute (larger, easier-to-measure draws). The peptide mass stays the same either way.
Worked example
Example
Peptide in vial5 mg
Bacteriostatic water added2 mL
Resulting concentration2.5 mg/mL
That is2,500 mcg/mL
If you instead added 1 mL, the same vial would be 5 mg/mL — twice as strong, so every dose draws half the volume.
How to use the calculator
- Enter the peptide amount printed on your vial (mg or mcg).
- Enter the bacteriostatic water you plan to add, in mL.
- Read off the concentration, then enter a desired dose to see the exact draw in insulin units.
Tip: if a dose comes out to only a unit or two on the syringe, add more water so it lands on a larger, more accurate volume.
Open the calculatorFrequently asked questions
How much bacteriostatic water should I add to a peptide vial?
There is no single correct amount — the volume of water sets the concentration, not the dose. Choose a volume that makes your typical dose easy to measure on a syringe. Adding more water dilutes the peptide so each dose draws a larger, easier-to-read volume; adding less makes a stronger, smaller-volume mix. The calculator shows the resulting concentration and draw volume for any amount you pick.
Does adding more water change the total amount of peptide?
No. The vial contains a fixed mass of peptide regardless of how much water you add. Water only changes how concentrated each millilitre is, which in turn changes the volume you draw for a given dose.
What is bacteriostatic water and why is it used?
Bacteriostatic water is sterile water containing a small amount of benzyl alcohol, which inhibits bacterial growth. That preservative is what allows a reconstituted multi-dose vial to be used over a period of time rather than discarded after one use.
Can I use regular sterile water instead?
Sterile water has no preservative, so it is generally intended for single use. Bacteriostatic water is the common choice for multi-use vials. This tool only does the math; follow appropriate guidance for the product you are using.