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How-to · reconstitution
Put 1 mL of bacteriostatic water into the 5 mg Tirzepatide vial. That makes 5.00 mg/mL. A 2.5 mg dose is then 50.0 units on a U-100 syringe.
That volume is the one that lands the draw on a syringe line, so the reading can be checked by eye. More water makes a weaker mix and a bigger draw. Less water makes a stronger mix and a smaller one. The amount of Tirzepatide in the glass never changes. Only the number of marks it takes does.
Tirzepatide is the drug in Zepbound and Mounjaro. It works on two gut hormone signals involved in appetite and blood sugar.
| Water added | Concentration | Per U-100 unit | Example: 2.5 mg = |
|---|---|---|---|
| 1 mL | 5.00 mg/mL | 50 mcg | 50.0 units |
| 2 mL | 2.50 mg/mL | 25 mcg | 100.0 units |
| 3 mL | 1.67 mg/mL | 16.67 mcg | 150.0 unitswon’t fit |
Straight arithmetic: concentration = 5 mg ÷ water, and 1 U-100 unit = 0.01 mL. The example column shows the math at 2.5 mg; it is not a dose recommendation.
Which volume to use is a practical choice rather than a dose. Pick the one that puts your draw somewhere you can actually read it on the syringe. The flags in the last column mark where that fails. “Hard to measure” means the draw is too small to judge by eye. “Won’t fit” means it is bigger than one U-100 syringe holds.
The table above uses one worked example. These are the published quantities, converted into marks on a U-100 barrel at each water volume for a 5 mg vial. Where a quantity was never studied by injection, the row says so.
2.5 mg once weekly for 4 weeks, under the skin.
| Water added | Concentration | Draw |
|---|---|---|
| 1 mL | 5.00 mg/mL | 50.0 units |
| 2 mL | 2.50 mg/mL | 100.0 units |
| 3 mL | 1.67 mg/mL | 150.0 unitswon’t fit |
FDA labelZepbound (tirzepatide) — FDA-approved Prescribing Information · retrieved 2026-07-22
5–15 mg once weekly, under the skin. (5, 10, or 15 mg).
| Water added | Concentration | Draw, low to high |
|---|---|---|
| 1 mL | 5.00 mg/mL | 100.0 to 300.0 unitswon’t fit |
| 2 mL | 2.50 mg/mL | 200.0 to 600.0 unitswon’t fit |
| 3 mL | 1.67 mg/mL | 300.0 to 900.0 unitswon’t fit |
FDA labelZepbound (tirzepatide) — FDA-approved Prescribing Information · retrieved 2026-07-22
At the worked amount, this vial holds two weekly doses and empties before the stated mixed-vial window closes. One of the three water options produces a draw too large for a U-100 syringe.
Spread this little powder through the largest listed volume and the solution gets very weak. One dose then needs more liquid than a U-100 syringe holds. The row stays in the table anyway, flagged.
Seeing why a volume fails is more useful than not seeing the volume. It also makes the direction obvious: on a small vial, adding water is the move that runs you out of barrel.
Two doses come out of this vial and both land inside the window the page gives for a mixed vial. That makes this the only tirzepatide size on the site with no wastage arithmetic to do.
Every larger vial on this ladder has the opposite problem, and the problem grows with the vial.
The labeled schedule increases over several months. At the worked amount, this vial holds two doses. At the highest labeled dose, it would not contain one full dose.
So the right vial size is not a fixed answer. It moves as the schedule moves, and this is the size that fits the beginning of it.
The table shows the offered water volumes. The calculator does the same arithmetic for any volume and any syringe, and shows its working line by line.
Open the Tirzepatide calculator →The arithmetic on this page is only half the question. These are the pages that carry the other half.