It happens — your first vial doesn't quite get you there. Take a breath. This calculator walks you through exactly how many units to draw from each vial, step by step.
Safety First: This calculator is for informational use only and does not replace your prescriber's instructions. Compounded GLP-1 medications can vary in concentration between pharmacies and even between batches. Always verify the concentration label on both vials before drawing. When in doubt, call your provider.
If you're on compounded semaglutide or tirzepatide, you may find yourself near the end of a vial but not quite at your full prescribed dose. This is completely normal — compounded vials don't always land you at exactly the right volume. The important thing is knowing how to finish your dose safely from a second vial.
The tricky part? Your two vials may have different concentrations. This is especially common when transitioning between compounding pharmacies, changing your dose tier, or when a pharmacy reformulates their product. You can't simply draw the same volume from vial B — you need to recalculate.
Needing two vials is common and expected. It doesn't mean you miscalculated — it just means your math needs to account for both vials.
Before calculating, confirm the concentration (mg/mL) printed on each vial's label. Don't rely on memory — check the label every time.
When a split dose requires two syringes, rotate your injection site for the second draw. Both injections deliver the same total dose.
If the concentrations are very different from what you expected, or if something doesn't look right on a label, call before injecting.
The calculator uses a simple two-step formula. First, it figures out how much of your dose Vial A can cover based on its remaining volume and concentration. Then it calculates exactly how much you need to draw from Vial B to reach your full prescribed dose — and converts everything into U-100 insulin syringe units.
All calculations happen in your browser. Nothing is sent to a server. Your dosing information is private.
The calculator will catch this and let you know — you won't need Vial B at all. This happens when the remaining volume in Vial A contains more mg than your prescribed dose.
U-100 syringes are the standard insulin syringes used for GLP-1 injections. The unit markings correspond to 0.01 mL per unit, so 100 units = 1 mL. This calculator assumes U-100 syringes.
Yes — the math is the same whether you're using compounded semaglutide or compounded tirzepatide. Just enter the concentrations from your specific vials.