Three different numbers
- mg is the amount of medicine. Your prescriber's dose is in mg.
- mL is the volume of liquid. The vial label gives the strength in mg per mL.
- Units are the marks on a U-100 insulin syringe: 100 units = 1 mL.
The formula
Units = dose (mg) ÷ strength (mg/mL) × 100. To go back: dose (mg) = units ÷ 100 × strength.
Example numbers only: 0.25 mg from a 2.5 mg/mL vial is 0.1 mL, which is 10 units. 5 mg from a 10 mg/mL vial is 0.5 mL, which is 50 units.
U-100 and U-40 syringes
A U-100 syringe has 100 unit marks per mL; a U-40 syringe has 40. The same mark is a different amount of liquid, so a units number is only right for the syringe type it was worked out for. Every number on this site is for U-100 insulin syringes.
Reading a compounded label
Look for the strength in mg/mL. Some labels give the total instead, such as "20 mg / 2 mL": divide to get the strength, here 10 mg per mL. If you mix a powder vial yourself, the strength is the mg in the vial divided by the mL of water added: 10 mg with 2 mL is 5 mg per mL. Follow your pharmacy's instructions.
Why the strength changes everything
Units only mean something for one strength. When a refill comes at a different mg/mL, the same number of units is a different dose. Check the label on every new vial and recalculate.
In July 2024 the FDA warned about dosing errors with compounded semaglutide, including confusion between mg, mL and units (FDA alert).