BAC water calculator
The amount of bacteriostatic water you add sets the concentration — and therefore how many units you draw. Try different BAC water volumes below to find one that keeps your dose easy to measure on an insulin syringe.
This calculator is a neutral conversion tool (mg ↔ solvent ↔ units). It makes no recommendation as to whether, what or how much to use, and does not replace medical advice. All inputs are your own.
How much bacteriostatic water to use
Bacteriostatic water is sterile water with 0.9% benzyl alcohol added, which lets you draw from the same vial repeatedly over several weeks. It is the standard diluent for reconstituting peptides.
The amount you add sets the concentration — and therefore how many units you draw for the same dose. More water means a lower concentration and a larger, easier-to-read draw. The table shows how it plays out for a 5 mg vial and a 250 mcg dose:
| BAC water | Concentration | Units to draw |
|---|---|---|
| 1 mL | 5 mg/mL | 5 U |
| 2 mL | 2.5 mg/mL | 10 U |
| 3 mL | 1.67 mg/mL | 15 U |
| 5 mL | 1 mg/mL | 25 U |
How to use the peptide calculator
- Enter the vial strength — the total milligrams of peptide in the vial.
- Enter the bacteriostatic (BAC) water you added, in millilitres.
- Enter your target dose in mcg or mg.
- Read the units to draw on a U-100 insulin syringe, plus the concentration and doses per vial.
Peptide reconstitution — frequently asked questions
How do I reconstitute a peptide?
Reconstitution means dissolving freeze-dried (lyophilized) peptide powder in bacteriostatic water. Add the water slowly down the vial wall and let it dissolve without shaking. Enter the vial strength, the BAC water you added and your target dose above, and the calculator shows the concentration and exactly how many units to draw.
How much bacteriostatic water should I add?
There is no single correct amount. More BAC water lowers the concentration and gives a larger, easier-to-read draw; less water raises the concentration and shrinks the draw. Choose an amount that keeps your dose within your syringe's range — change the BAC water value above to see the units update live.
How many units do I draw on an insulin syringe?
On a U-100 insulin syringe, 100 units equal 1 mL. The calculator converts your dose and concentration into units so you can read them straight off the syringe barrel. The figure above updates instantly as you change the inputs.
How do I convert mcg to units?
Units depend on the concentration, not the dose alone — the same dose in mcg draws to different units depending on how much BAC water you used. Enter your dose in mcg or mg with the vial and water values, and the calculator returns the exact units to draw.
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