Bacteriostatic water calculator
Enter the vial amount, your prescribed dose and the number of syringe units you want that dose to fill. Get the mL of water to add and the concentration that results.
Enter the vial amount, your prescribed dose and the units you want that dose to fill.
Arithmetic only. This tool does not recommend a dose or a schedule; use the ones your prescriber gave you.
The formula
The usual calculator starts from the water and tells you the units. This one runs the same arithmetic backwards: you pick the units and it tells you the water. If you want 10 units (0.1 mL) to hold 0.5 mg, the concentration has to be 5 mg/mL, and a 10 mg vial reaches 5 mg/mL with 2 mL of water.
To check a result, put the water figure into the reconstitution calculator. It should return the units you chose here.
Questions
How much bacteriostatic water do I add to a vial?
There is no single correct volume. The water sets the concentration, and the concentration sets how many units your dose takes up on the syringe. Follow the volume on your prescription or pharmacy label if one is given. If the choice is yours, pick a draw size that is easy to read and this calculator returns the water that produces it.
Does the amount of water change how much is in the vial?
No. The vial holds the same amount whatever you add. More water makes a weaker solution, so the same dose becomes a larger draw. Less water makes a stronger solution and a smaller draw.
What if the answer is more water than the vial can hold?
Choose a smaller number of units. Halving the units halves the water. Vials differ in size, so check the capacity of yours before adding anything.
Why would I want the dose to fill a round number of units?
Syringe markings are easier to read on a line than between lines. A 1 mL syringe is usually marked every 2 units, so a draw of 10 or 20 units sits exactly on a marking.