What this calculator does
If you’re prescribed compounded semaglutide or tirzepatide in a vial, this tool converts your dose in milligrams into the exact units to draw on a U-100 (insulin) syringe. Brand pens dial the dose for you; a vial doesn’t, so you have to translate “0.25 mg” into a mark on the barrel — and that translation depends entirely on how concentrated your vial is.
It’s a converter, not a dosing recommendation. It assumes you already have a prescribed dose and a vial with a known, labeled concentration.
Why milligrams alone aren’t enough
The single most common mistake with compounded GLP-1s is treating milligrams as if they map directly to units. They don’t. The same dose lands at different places on the syringe depending on the vial’s concentration:
| Your dose | At 2.5 mg/mL | At 5 mg/mL | At 10 mg/mL |
|---|---|---|---|
| 0.25 mg | 10 units | 5 units | 2.5 units |
| 0.5 mg | 20 units | 10 units | 5 units |
| 2.5 mg | — | 50 units | 25 units |
| 5 mg | — | — | 50 units |
Read the concentration off your vial or pharmacy label every time, and enter it above. If you reconstituted the vial yourself, the concentration is the total mg divided by the mL of water you added — the peptide reconstitution calculator works that out.
How the math works
Volume (mL) = dose (mg) ÷ concentration (mg/mL) Units = volume (mL) × 100
Example — a 0.5 mg semaglutide dose from a 2.5 mg/mL vial:
- Volume: 0.5 ÷ 2.5 = 0.2 mL
- Units: 0.2 × 100 = 20 units
You’d draw to the 20-unit mark.
Pens vs. vials — which do you have?
This matters, so be sure which product you’re using:
- Brand pens (Ozempic, Wegovy, Zepbound, Mounjaro) — you dial the dose; there’s nothing to convert. Follow the pen instructions and the titration schedule on our dosage charts.
- Compounded vials — a pharmacy fills a multi-dose vial and you draw each dose with a syringe. That’s what this calculator is for.
Important safety notes
- This does not set your dose. Use only the dose your clinician prescribed, and follow the titration schedule they gave you.
- Compounded medications are not FDA-approved for safety, effectiveness, or quality, and rules on compounding tightened after the branded shortages ended. If you use a compounded GLP-1, use a licensed provider and pharmacy.
- Confirm the labeled concentration with your pharmacy before drawing, and double-check the calculated units against their instructions.
- If a result looks surprising — far more or fewer units than usual — stop and verify the concentration rather than injecting.
This tool is general education, not medical advice.