Reconstitution calculator
Updated 24 October 2026
Some prescription injectables arrive as a powder that you mix with a liquid, usually bacteriostatic water, before the first dose. Once it is mixed, the vial has a concentration in mg per mL, and every dose you draw depends on that number. This calculator gives you both.
Only mix a vial your pharmacy or prescriber told you to mix, with the liquid and the volume they specified. Many vials, Zepbound's among them, arrive already as a solution and must not be diluted.
Calculator
Concentration: 2.5 mg/mL
Draw 10 units (0.1 mL)
0.25 mg at 2.5 mg/mL on a U-100, 1 mL (100 units) syringe. Round to the nearest mark your syringe has.
Draw to 10 units
U-100, 1 mL (100 units) insulin syringe
Free, no account needed. Your vials stay on this device.
How reconstitution math works
- Concentration = powder (mg) ÷ water added (mL). A 5 mg vial mixed with 2 mL is 2.5 mg/mL.
- Volume to draw = dose (mg) ÷ concentration (mg/mL). A 0.25 mg dose at 2.5 mg/mL is 0.1 mL.
- Units on a U-100 syringe = volume (mL) × 100. 0.1 mL is 10 units.
Adding more water doesn't change the dose in the vial, only how spread out it is. More water means more units for the same mg, which can make small doses easier to measure, as long as the dose still fits in your syringe.
Concentration chart
Concentration after mixing, and how much drug every 10 units (0.1 mL) on a U-100 syringe holds.
| Powder in vial | + 1 mL water | + 2 mL water | + 3 mL water |
|---|---|---|---|
| 2 mg | 2 mg/mL · 10 units = 0.2 mg | 1 mg/mL · 10 units = 0.1 mg | 0.667 mg/mL · 10 units = 0.067 mg |
| 5 mg | 5 mg/mL · 10 units = 0.5 mg | 2.5 mg/mL · 10 units = 0.25 mg | 1.667 mg/mL · 10 units = 0.167 mg |
| 10 mg | 10 mg/mL · 10 units = 1 mg | 5 mg/mL · 10 units = 0.5 mg | 3.333 mg/mL · 10 units = 0.333 mg |
| 15 mg | 15 mg/mL · 10 units = 1.5 mg | 7.5 mg/mL · 10 units = 0.75 mg | 5 mg/mL · 10 units = 0.5 mg |
Write the concentration on the vial
After mixing, write the concentration and the date on the vial label. Next week you will not remember whether you added 2 mL or 3 mL, and drawing the units for the wrong one gives you a third less, or half as much again. Ask your pharmacy how long the mixed vial keeps and how to store it.