Concentration is set the moment you add the water
The strength of the finished solution depends only on the powder in the vial and how much water you add. Once you know the concentration in mcg per mL, the volume for any dose is a single division.
Concentration by vial size and water added
More water gives a weaker, more dilute solution, so each dose takes a larger, easier-to-measure volume. Less water concentrates it, which suits very small doses.
| Vial | BAC water | Concentration | 250 mcg dose |
|---|---|---|---|
| 5 mg | 1 mL | 5,000 mcg/mL | 0.05 mL = 5 units |
| 5 mg | 2 mL | 2,500 mcg/mL | 0.10 mL = 10 units |
| 10 mg | 1 mL | 10,000 mcg/mL | 0.025 mL = 2.5 units |
| 10 mg | 2 mL | 5,000 mcg/mL | 0.05 mL = 5 units |
| 20 mg | 2 mL | 10,000 mcg/mL | 0.025 mL = 2.5 units |
The vial holds the same total either way. Choosing 2 mL over 1 mL does not change the total; it just spreads each dose across twice the volume, which is easier to draw accurately.
Reading the syringe
- U-100 scale. On a standard insulin syringe, 100 units = 1 mL, so 1 unit is 0.01 mL. Multiply your volume in mL by 100 to get units.
- Milligrams vs micrograms. Vials are labelled in mg, doses in mcg, and 1 mg = 1,000 mcg. Convert first, then divide, or the dose will be off by a factor of 1,000.
- Bacteriostatic, not sterile, water. The benzyl alcohol preservative lets a vial be reused for roughly 28 days refrigerated. Sterile water has no preservative and is single-use.
Common questions
How do I work out the injection volume for a dose?
Divide the dose you want by the concentration. If the vial is at 5,000 mcg/mL and you want 250 mcg, that is 250 / 5,000 = 0.05 mL, which reads as 5 units on a U-100 insulin syringe.
Why use bacteriostatic water instead of sterile water?
Bacteriostatic water contains about 0.9% benzyl alcohol, which holds off bacterial growth so a multi-dose vial can be punctured repeatedly over several weeks. Plain sterile water has no preservative and is only meant for single use.
What does the mcg vs mg mistake look like?
A vial is labelled in milligrams but doses are usually written in micrograms, and there are 1,000 mcg in 1 mg. Reading a 250 mcg target as 250 mg would be a 1,000-fold error, so always confirm which unit the dose is in.
