A U-100 syringe is calibrated so that 100 units = 1 mL. “Units” on the barrel are a measure of volume, not of medication strength. How much drug is in a unit depends on the concentration of your vial.
Syringe sizes
| Syringe | Holds | Each small line usually equals |
|---|---|---|
| 0.3 mL | 30 units | 0.5 or 1 unit |
| 0.5 mL | 50 units | 1 unit |
| 1 mL | 100 units | 2 units |
Check the markings on your own syringe, since line spacing differs by size.
Converting units
- Units → mL: divide by 100. 20 units = 0.2 mL.
- mL → mg: multiply by concentration. 0.2 mL × 2.5 mg/mL = 0.5 mg.
- mg → units: (mg ÷ concentration) × 100.
The reconstitution calculator does all three steps for you.
Mistakes that cause dosing errors
- Mixing up mL and units. Drawing “0.25” on a units-marked syringe gives you almost nothing, while drawing 25 units when you meant 2.5 gives you ten times the dose. The FDA has documented both kinds of confusion with compounded semaglutide.
- Assuming every vial has the same strength. A refill from a different pharmacy can be a different concentration.
- Reading the wrong edge of the plunger. Read the dose at the flat top edge of the black plunger stopper, not the tip.
- Using a syringe that isn’t U-100. U-40 and U-500 syringes exist; the math here won’t apply.
If anything doesn’t add up, stop and call your pharmacist before injecting.