Compute exact U-100 syringe draw volumes when your prescribed dose spans two vials with different concentrations. Full formula output, no approximations.
Clinical Note: Compounded GLP-1 medications (semaglutide, tirzepatide) may vary in concentration between compounding pharmacies and between production batches. Verify the mg/mL concentration on each vial's label before entering values. This calculator does not validate that entered concentrations match your prescription — cross-reference your Rx before injecting.
mg_in_A = vol_A × conc_Aif mg_in_A ≥ prescribed_dose → draw only from Vial Aremaining_mg = prescribed_dose − mg_in_Avol_B = remaining_mg ÷ conc_Bunits_A = vol_A × 100 [U-100 basis]units_B = vol_B × 100 [U-100 basis]| Parameter | Value | Notes |
|---|---|---|
| Prescribed dose | 5 mg | from Rx |
| Vial A conc. | 6 mg/mL | from label |
| Vial A remaining | 0.300 mL | visual or calculated |
| mg from Vial A | 1.80 mg | 0.3 × 6 |
| Units from Vial A | 30 units | 0.3 mL × 100 |
| Remaining mg | 3.20 mg | 5 − 1.8 |
| Vial B conc. | 9 mg/mL | from label |
| Vol from Vial B | 0.356 mL | 3.2 ÷ 9 |
| Units from Vial B | 36 units | 0.356 mL × 100 (rounded) |
The split-dose scenario arises when a patient's prescribed GLP-1 dose cannot be completed from a single vial. This occurs most frequently with compounded semaglutide and tirzepatide when the residual volume in an active vial contains insufficient medication to fulfill the full prescribed milligram dose. The complicating variable is that the replacement vial may carry a different concentration — which invalidates a simple volume-based continuation of the draw.
A common error in split-dose situations is assuming that the remaining milliliters from Vial A can be added directly to an equal or proportional draw from Vial B. This is only valid if both vials share an identical concentration. When concentrations differ — as is common when transitioning between compounding pharmacies, dose tiers, or pharmacy reformulations — each vial requires its own volume-to-units calculation based on its specific mg/mL value.
This calculator outputs draw volumes in U-100 insulin syringe units. The U-100 system defines 1 mL as 100 units, meaning each unit corresponds to 0.01 mL. Volume-to-units conversion: units = volume (mL) × 100. This applies uniformly regardless of medication concentration — the syringe unit scale reflects volume only; the mg value delivered is determined by the concentration of the medication in that volume.
Unit values are rounded to the nearest whole unit (0.01 mL). For most GLP-1 dose ranges this introduces a rounding delta of ≤0.005 mL, which is clinically negligible. Volume values are displayed to three decimal places for transparency. When absolute precision is required, verify the unrounded volume against your syringe's minimum graduation.
Vial A contains sufficient dose: If mg_in_A ≥ prescribed_dose, the calculator will indicate that only Vial A is needed and calculate the reduced draw volume. Single-vial result is returned — Vial B is not used. This commonly occurs when the residual volume exceeds what is needed at a lower concentration.
Very high unit counts (>100u): When the calculated draw from either vial exceeds 100 units, the syringe visual flags this. A standard 1 mL U-100 syringe accommodates 100 units maximum. Verify with a larger syringe or consult your pharmacist.
Concentration mismatch between Rx and vial label: This calculator does not validate that entered concentrations match your prescription. If your vial's label concentration differs from what your prescriber specified, do not inject — contact your compounding pharmacy for clarification.