Guide
How reconstitution math works
Three formulas turn the numbers on a vial into a line on a syringe.
Step 1: Convert everything to the same unit
Most errors come from mixing mg and mcg. Convert both to mcg first.
1 mg = 1,000 mcg
Step 2: Find the concentration
Divide the total amount in the vial by the volume of bacteriostatic water you added.
concentration (mcg/mL) = vial amount (mcg) ÷ water (mL)
Step 3: Find the volume to draw
volume (mL) = amount per draw (mcg) ÷ concentration (mcg/mL)
Step 4: Convert volume to syringe units
units = volume (mL) × 100 (U-100 syringe)
units = volume (mL) × 40 (U-40 syringe)
units = volume (mL) × 40 (U-40 syringe)
Worked examples
These examples show the arithmetic only. They are not recommended amounts.
| Vial | Water | Concentration | Amount | Volume | U-100 units |
|---|---|---|---|---|---|
| 5 mg | 2 mL | 2,500 mcg/mL | 250 mcg | 0.1 mL | 10 |
| 5 mg | 2.5 mL | 2,000 mcg/mL | 500 mcg | 0.25 mL | 25 |
| 10 mg | 2 mL | 5,000 mcg/mL | 1000 mcg | 0.2 mL | 20 |
| 2 mg | 1 mL | 2,000 mcg/mL | 100 mcg | 0.05 mL | 5 |
Why water volume matters
Adding more water doesn't change the total amount in the vial. It spreads it across more liquid, so each unit on the syringe contains less. Choosing a water volume that puts your draw in an easy-to-read range on the syringe makes measuring more precise.