How much bacteriostatic water to add
There is no single right amount. The water only sets how concentrated the vial is. Pick an amount that puts your dose on a line that is easy to read, usually between 10 and 50 units.
Water (mL) = vial mg × target units ÷ (dose mg × 100)
Example: a 10 mg vial and a 0.5 mg dose that you want at 20 units: 10 × 20 ÷ (0.5 × 100) = 4 mL. That is more than many vials hold, so you could aim for 10 units instead: 2 mL.
Water for a 10 mg vial
Water that puts the dose at 10, 20 or 25 units:
| Dose | 10 units | 20 units | 25 units |
|---|---|---|---|
| 0.25 mg | 4 mL | 8 mL | 10 mL |
| 0.5 mg | 2 mL | 4 mL | 5 mL |
| 1 mg | 1 mL | 2 mL | 2.5 mL |
| 2 mg | 0.5 mL | 1 mL | 1.25 mL |
| 2.5 mg | 0.4 mL | 0.8 mL | 1 mL |
| 5 mg | 0.2 mL | 0.4 mL | 0.5 mL |
Rules of thumb
- More water means more units per dose and easier reading, but the vial has to hold it. Check the vial size.
- Less water means fewer units per dose. Doses under 5 units are hard to read on a 1 mL syringe.
- Write the mg and mL on the vial label the day you mix it.
Already mixed it?
Tell us the vial, the water and your dose, and see the units on your syringe.
Calculate my unitsQuestions
Does more water make the dose weaker?
No. The dose in mg stays the same. More water means you draw more units to get it.
How much water fits in a vial?
It depends on the vial. Many hold 3 mL or more, but check yours before mixing.