Bacteriostatic water calculator
Choose the syringe mark you want each dose to fill, and this calculator works backwards to how much bacteriostatic water to add to the vial — and the concentration that gives you.
The syringe mark you want each dose to fill, e.g. 10 units.
Bacteriostatic water to add
2mL
- Concentration
- 2.5 mg/mL
- per unit
- 25 mcg
- Volume per dose
- 0.1 mL
- Doses per vial
- ~20
How much bacteriostatic water should you add?
There’s no single correct volume — it’s a trade-off. More water makes each dose a larger draw that’s easier to read; less water keeps the volume per dose small. Working backwards from a round syringe mark (5, 10 or 20 units) means every dose lands on a printed line instead of between two.
The math
Volume per dose (mL) = units ÷ units per mL on the syringe (100 on U-100, 40 on U-40). Concentration (mg/mL) = dose in mg ÷ volume per dose. Water to add (mL) = peptide in the vial (mg) ÷ concentration. Example: 5 mg vial, 250 mcg dose, 10 units on U-100 → 0.10 mL per dose → 2.5 mg/mL → 2 mL of water.
Worked examples
Each row is calculated with the formula above. These are unit-conversion results, not dose recommendations.
| Peptide in vial | Dose | Units per dose | Concentration | Doses per vial | Water to add |
|---|---|---|---|---|---|
| 5 mg | 250 mcg | 10 units (U-100) | 2.5 mg/mL | 20 | 2 mL |
| 5 mg | 250 mcg | 20 units (U-100) | 1.25 mg/mL | 20 | 4 mL |
| 10 mg | 500 mcg | 10 units (U-100) | 5 mg/mL | 20 | 2 mL |
| 15 mg | 1000 mcg | 20 units (U-100) | 5 mg/mL | 15 | 3 mL |
| 5 mg | 500 mcg | 25 units (U-100) | 2 mg/mL | 10 | 2.5 mL |
| 5 mg | 250 mcg | 10 units (U-40) | 1 mg/mL | 20 | 5 mL |
Frequently asked questions
How much bacteriostatic water should I add to a peptide vial?
It depends on the syringe mark you want each dose to fill. Water (mL) = vial mg ÷ concentration, where concentration = dose in mg ÷ (units ÷ 100) on a U-100 syringe. For a 5 mg vial, a 250 mcg dose and a 10-unit draw that is 5 ÷ 2.5 = 2 mL. Most references use 1–3 mL per vial.
Why choose the units first?
Round marks such as 5, 10 or 20 units are easier to read on an insulin syringe than values that fall between lines. Fixing the mark first and solving for the water volume means each dose lands exactly on a printed line.
What happens if I add too much or too little water?
The amount of peptide stays the same either way. Too little water makes a strong solution: each unit holds more peptide, so a small misreading is a bigger error, and the powder may not dissolve fully. Too much water makes each draw larger and may not fit in the vial — the calculator warns above 10 mL, more than most peptide vials hold.
Does it matter whether I use a U-100 or a U-40 syringe?
Yes. A U-40 syringe marks 40 units per mL, so each unit is 2.5 times the volume of a U-100 unit. For the same dose and number of units you need 2.5 times as much water — 5 mL instead of 2 mL in the example above. 1 mL, 0.5 mL and 0.3 mL insulin syringes are all U-100, so they give the same answer; the smaller barrels just hold fewer units.
Can I change the water volume after reconstituting?
Only upwards. You can add more water to dilute the solution further — then recalculate the units per dose with the reconstitution calculator — but water can’t be taken back out. Settle on the volume before adding any.