About this page

This page is for people who have been prescribed tirzepatide and need to calculate the volume to draw. The calculator above does the math. The content below explains how to use it correctly and cites the FDA-approved labels for the branded products. Tirzepatide is FDA-approved as Mounjaro for type 2 diabetes and as Zepbound for chronic weight management.¹²

About tirzepatide

Tirzepatide is a dual GIP and GLP-1 receptor agonist developed by Eli Lilly. The FDA approved Mounjaro in May 2022 for type 2 diabetes and Zepbound in November 2023 for chronic weight management in adults with obesity or overweight with at least one weight-related comorbidity.¹²

In branded form, both Mounjaro and Zepbound ship as prefilled, single-dose pen injectors. The pen device delivers the prescribed dose with no reconstitution required. Compounded tirzepatide, prescribed by some telehealth providers, is typically dispensed as a lyophilized powder in a multi-dose vial that requires reconstitution with bacteriostatic water before use. The calculator on this page is for the compounded form.

The FDA-approved dosing ladder

The Zepbound label specifies a starting dose of 2.5 mg administered subcutaneously once weekly. After 4 weeks, the dose is increased to 5 mg once weekly. Subsequent dose escalations to 7.5 mg, 10 mg, 12.5 mg, and 15 mg once weekly may occur at intervals of at least 4 weeks. The label notes maintenance doses of 5 mg, 10 mg, or 15 mg once weekly.¹

The Mounjaro label specifies the same starting dose of 2.5 mg once weekly for 4 weeks, with subsequent increases in 2.5 mg increments at intervals of at least 4 weeks, up to a maximum of 15 mg once weekly. The intermediate doses are 5 mg, 7.5 mg, 10 mg, 12.5 mg, and 15 mg.²

Both labels frame the 2.5 mg starting dose as a tolerability dose, not a therapeutic target. The labels are the authoritative reference for dosing decisions; this page restates them for context and does not provide an independent dosing schedule. If your prescription differs from the FDA label, your prescriber has reasons that take precedence over the label’s general guidance.

Reconstitution math for compounded tirzepatide

Compounded tirzepatide vials are commonly dispensed in total mg quantities of 10 mg, 15 mg, 20 mg, or 30 mg. The bacteriostatic water volume used for reconstitution varies by pharmacy, and the resulting concentration depends on both inputs. Common reconstitutions produce concentrations of 5 mg/mL or 10 mg/mL, but pharmacies vary.

The math itself is unchanged regardless of the compound. The formulas from the main calculator:

Concentration = vial mg ÷ water mL
Volume to draw = target dose mg ÷ concentration
Units = volume mL × syringe scale (100 for U-100, 40 for U-40)

Example calculation (not a recommendation): If your prescription is for 2.5 mg weekly and your pharmacy provides a 10 mg vial reconstituted with 2 mL of bacteriostatic water, the calculator computes a concentration of 5 mg/mL. A 2.5 mg dose at that concentration requires a 0.5 mL draw, which reads as 50 units on a U-100 syringe.

Second example (not a recommendation): Once a patient has titrated up to 5 mg weekly on that same 10 mg / 2 mL vial, the concentration is unchanged at 5 mg/mL. A 5 mg dose now requires a 1.0 mL draw, which reads as 100 units on a U-100 syringe — the full barrel of a 1 mL insulin syringe. Some patients at higher doses on this concentration split the injection or reconstitute with less water for future vials to keep the draw under 100 units.

Third example (not a recommendation): A 7.5 mg weekly dose on a 30 mg vial reconstituted with 3 mL of water produces a concentration of 10 mg/mL. A 7.5 mg dose at that concentration requires a 0.75 mL draw, which reads as 75 units on a U-100 syringe. Higher concentrations mean lower draw volumes at the same dose.

The three examples illustrate how the draw volume scales with both the dose and the reconstitution choice. Always confirm the vial size and water volume listed on your pharmacy paperwork before drawing.

Branded versus compounded

Branded tirzepatide — Mounjaro and Zepbound — ships in single-dose prefilled pens. The pen device handles dose delivery internally. Users do not need to reconstitute, calculate volume, or measure with a syringe. The calculator on this page does not apply to branded prefilled pens.

Compounded tirzepatide is prescribed by some telehealth providers when branded products are unavailable, when insurance does not cover the branded form, or when a specific dose is needed that the branded pens do not offer. Compounded products may come from 503A pharmacies (patient-specific compounds prepared on receipt of a prescription) or, in some cases, 503B outsourcing facilities (larger-scale sterile compounding under FDA registration). The regulatory framework for each pharmacy type differs and is worth understanding if you have not encountered compounded medications before.

This page makes no recommendation between branded and compounded forms. That decision belongs to you and your prescriber.

Beyond-use date considerations

Reconstituted tirzepatide is constrained by the same two clocks as any reconstituted peptide.

Bacteriostatic water has a 28-day window after first puncture under USP standards.³ The benzyl alcohol preservative is considered effective against microbial growth for up to 28 days following the first needle entry, and the bottle should be discarded after that point.

Tirzepatide stability in solution after reconstitution is less consistently reported. The branded Mounjaro and Zepbound products are not designed for reconstitution by end users — they are formulated as ready-to-administer pens — so the published stability data for those products does not translate directly to compounded vials. Compounding pharmacies determine BUDs for their own products based on their formulation, storage conditions, and validation studies. The pharmacy’s BUD, if specified, is the authoritative reference for that product.

In practice, track three dates: the bacteriostatic water first-puncture date, the reconstitution date, and any pharmacy-specified BUD. The earliest of the three is your effective BUD. The longer explanation of how these windows interact is in The 28-day rule for bacteriostatic water.

Math, not medical advice

Joust doesn’t recommend doses, prescribe protocols, or claim therapeutic benefits. This page is a calculator, not a prescription. Your prescriber determined your dose and your titration schedule. If those don’t match what your label or pharmacy instructions say, contact them directly.

The calculator’s outputs are correct for the inputs given. The correctness of the inputs is your responsibility. The labels cited on this page describe FDA-approved dosing for the branded products and do not constitute medical advice for any individual.

When to talk to your prescriber

Contact your prescriber if:

Your prescribed dose has changed
You have missed doses or experienced side effects
You are switching from branded to compounded tirzepatide or vice versa
You are receiving compounded product from a new pharmacy with a different formulation
The calculator output does not match the instructions on your pharmacy paperwork
You experience symptoms that may be related to the medication

For escalation timing, dose adjustments, and any medical concerns, your prescriber’s guidance applies. The FDA labels referenced on this page describe general dosing intervals but do not address individual circumstances.

References

U.S. Food and Drug Administration. Zepbound (tirzepatide) Prescribing Information. Eli Lilly and Company. ↩ ↩² ↩³
U.S. Food and Drug Administration. Mounjaro (tirzepatide) Prescribing Information. Eli Lilly and Company. ↩ ↩² ↩³
United States Pharmacopeial Convention. USP General Chapter <797>: Pharmaceutical Compounding — Sterile Preparations. Establishes maximum beyond-use dates for multi-dose preserved sterile containers, including the 28-day window for bacteriostatic water after first puncture. ↩