The peptide dosage formula
Units to draw = dose (mcg) × water (mL) ÷ vial (mg) ÷ 10. A 500 mcg dose from a 10 mg vial mixed with 2 mL is 500 × 2 ÷ 10 ÷ 10 = 10 units on a U-100 syringe, which is 0.10 mL.
The ÷ 10 folds three steps into one: × 1,000 to turn the vial into mcg, and × 100 to turn millilitres into U-100 units, because a U-100 barrel is marked at 100 units per mL. The insulin syringe unit converter covers that scale. Written out in full, it is dose ÷ (vial mcg ÷ water mL) × 100.
Dose, volume, and units are three views of one number
A dose is an amount of peptide, usually written in micrograms. A syringe cannot measure micrograms; it measures volume. So a dose only becomes drawable once the concentration of the vial is known, and the concentration depends entirely on how much bacteriostatic water was added.
This is why the same 500 mcg dose is a different mark on the syringe for two people holding identical vials. The dose has not changed. The dilution has.
The same 500 mcg dose from a 10 mg vial, at three different dilutions.
| Water added | Concentration | Volume to draw | U-100 units |
|---|---|---|---|
| 1 mL | 10 mg/mL | 0.05 mL | 5 units |
| 2 mL | 5 mg/mL | 0.10 mL | 10 units |
| 3 mL | 3.33 mg/mL | 0.15 mL | 15 units |