Bacteriostatic Water Calculator
How much bacteriostatic water to add is the one genuine decision in reconstitution, and it is permanent — concentration is fixed the moment the water goes in. Enter your vial size and dose, then try different water volumes to see how each one changes the units you would draw and the doses the vial holds.
100 units per mL — pick the syringe you have.
Draw to
BAC water
on a U-100 insulin syringe
Drag the syringe to set your dose, or type the values above.
Enter your vial amount and dose, then type the bacteriostatic water or drag the syringe to see the exact units to draw.
Vial sizes and water volumes are shown for convenience — the dose field is deliberately not prefilled and has no suggestions. Enter your own numbers.
Never do this math twice.
The Pep AI app remembers your vials and doses, shows an interactive syringe you draw to, counts down the doses left in each vial, and reminds you when it's time. Free on iOS and Android.
How water volume changes everything downstream
Adding water does not change what is in the vial — it changes how spread out it is. The example uses an arbitrary dose:
1. Concentration
Vial amount ÷ water added. A 5 mg vial with 2 mL of water is 2.5 mg/mL.
2. Volume per dose
Your dose ÷ concentration. A 0.5 mg dose at 2.5 mg/mL is 0.2 mL.
3. Units to draw
Volume per dose × 100 for a U-100 syringe (or × 40 for a U-40). 0.2 mL on a U-100 syringe is 20 units.
Worked example
You have a 5 mg vial, you want a 500 mcg dose, and you add 2 mL of bacteriostatic water. Concentration is 5 ÷ 2 = 2.5 mg/mL. Volume per dose is 0.5 ÷ 2.5 = 0.2 mL. On a U-100 syringe that's 0.2 × 100 = 20 units, and the vial holds 10 doses.
The trade-off, in both directions
More water means a weaker solution, so any given dose occupies a larger volume and reads as a bigger number of syringe units. Large numbers are easier to draw accurately — a one-unit misread is a smaller proportional error at thirty units than at three. The costs are a larger injection volume and fewer total doses in the vial.
Less water means the opposite: small comfortable volumes, more doses per vial, but each unit mark now carries more compound, so the same misread costs proportionally more.
There is no correct answer, only a sensible target — pick a volume that puts your dose somewhere in the middle of the syringe barrel rather than crammed into the first few marks. Try a few values above before committing, because once the water is in, this is settled.
Frequently asked questions
This calculator is provided for informational and educational purposes only and is not medical advice. Always confirm dosing with a qualified healthcare professional and double-check your own measurements before administering anything.
Built by the Pep AI team.