Why "units" and "mg" are not the same thing
Milligrams (mg) measure the amount of drug. Units, on an insulin syringe, measure a volume: a U-100 syringe is marked so that 100 units equals 1 milliliter (mL). How many milligrams sit in that volume depends entirely on how concentrated the liquid is — the mg/mL figure printed on the vial. Two pharmacies can dispense compounded semaglutide at different concentrations, so the same number of units can be a very different dose.
The formula
For a U-100 syringe: units = (dose in mg ÷ concentration in mg/mL) × 100, and the volume you draw is dose in mg ÷ concentration in mg/mL (in mL). To go from units back to mg: mg = (units ÷ 100) × concentration in mg/mL. The calculator above does this both ways.
Where dosing errors come from
The most common mistake is using the wrong concentration — reading a number off an old vial, a different pharmacy's vial, or assuming a "standard" strength that doesn't exist. Always read the concentration off the vial in your hand. If a converted dose comes out to more than about half a milliliter (50 units) in a single injection, that is unusual for compounded GLP-1 and usually a sign the concentration was entered wrong. When in doubt, do not inject — call your pharmacy. For the FDA-labeled dosing of the brand products, see our semaglutide dosage guide and tirzepatide dosage guide, and read why compounded is different in our compounded safety guide.