The principle in one sentence
Concentration = active substance in the vial ÷ liquid. A 10 mg vial stays a 10 mg vial – whether you add 1 mL, 2 mL or 3 mL. The only thing that changes is how much liquid the 10 mg is spread across.
| 10 mg dissolved in | Concentration | 1 mg equals |
|---|---|---|
| 1 mL | 10 mg/mL | 0.10 mL = 10 units |
| 2 mL | 5 mg/mL | 0.20 mL = 20 units |
| 3 mL | 3.33 mg/mL | 0.30 mL = 30 units |
The table is purely a calculation example, not a dosing instruction.
Which amount is practical?
- Too little water: The solution is so concentrated that even small reading errors (1–2 units) make a big difference.
- Too much water: You have to draw up and inject large volumes; with pens, the cartridge (usually 3 mL) is the upper limit.
- Rule of thumb: Choose the amount so that your application lands on a round marking and not below about 5 units. That is exactly what our Calculator works out under “Round units”.
The most common misunderstandings
- “You mix everything with 3 mL” – no, you can choose the amount freely; it just has to match your calculation.
- “0.5 mL can’t be right” – yes it can, if the calculator says so. What matters is that you then keep calculating with exactly that concentration.
- Adding water afterwards without recalculating – then every amount after that is wrong.
- Not writing down the concentration – after two weeks nobody remembers how the vial was mixed. Our Tracker saves it and prints you a label.
When to see a doctor
- You are unsure whether your calculation is right – have it double-checked at a pharmacy
- Unusually strong side effects occur after an application – the amount may have been wrong
In an emergency: call 112.
FAQ
Does more water change the effect?
No. The amount of active substance in the vial stays the same. More water only dilutes it; you draw up more volume for the same amount.
How much bacteriostatic water do I need for 10 mg?
That depends on what amount you want to measure. Choose the amount of water so that your amount is easy to read. The reconstitution calculator shows suitable options.
Can I add more water later?
Technically yes, but then the concentration changes and every further calculation has to be redone. Better to plan it properly from the start.
This article does not replace medical advice. It does not give any dosing recommendations. As of September 2026.