GLP-1 Reconstitution Calculator

The math

The three identities are universal:

• Concentration (mg/mL) = vial mg ÷ BAC water mL
• Volume per dose (mL) = target dose mg ÷ concentration
• Units on a U-100 syringe = volume × 100 (because U-100 means 100 units per mL)

U-100 is the FDA-recognized specification, and it fixes one thing: 100 units fill 1 mL, so a single unit is 0.01 mL. Every mark on the barrel measures volume, which is why a “5 unit” mark is 0.05 mL of liquid and says nothing at all about how much drug is dissolved in it.

Why this calculator has hard limits

Some combinations of inputs produce an answer nobody should act on. Those either stop the calculation or raise a warning you cannot miss:

• FDA maximum weekly dose [3, 4, 5]: nothing above the labeled weekly maximum will compute at all — 2.4 mg for semaglutide as Wegovy, 15 mg for tirzepatide as Mounjaro or Zepbound.
• Below 6 units (0.06 mL): below a fifth of the barrel, a U-100 syringe stops measuring accurately [10]. You get told, and the fix offered is less water and a stronger solution.
• Above 50 units (0.5 mL): is more than anyone wants going under the skin in one weekly injection. Again the answer is less water.
• Above 1.0 mL: will not fit in one draw on any standard U-100 syringe. That one stops you.
• Figures that look wrong. A vial over 200 mg, or more than 50 mL of water, asks you to check what you typed before it continues.

Bacteriostatic water and beyond-use dating

Bacteriostatic water for injection (USP) [2] contains 0.9% benzyl alcohol as a preservative. After the vial is first punctured, the standard pharmacy practice is a 28 days in use, kept at controlled room temperature, and then the vial goes.

The reconstituted GLP-1 vial itself has its own beyond-use date (BUD), assigned by the compounding pharmacy based on their sterility testing and environmental monitoring per USP <797> [1]. Category 1 (low-risk) sterile preparations can be assigned a BUD up to 60 days refrigerated under the 2023 revision of USP <797>. The actual labeled BUD on your specific vial is the authoritative number — follow your pharmacy's labeled BUD, not a generic estimate.

Compounded GLP-1 legal status (April 2026)

The FDA removed tirzepatide from its drug shortage list on October 2, 2024, with 503A enforcement discretion ending February 18, 2025 [7]. Semaglutide was removed on February 21, 2025, with the 503A grace period running through April 22, 2025 [8]. As of April 2026, neither compound is under FDA enforcement discretion for routine 503A compounding unless the patient has a documented medical need that requires a formulation change (e.g., a documented allergy to an inactive ingredient in the brand product). Verify your pharmacy's legal status before ordering.

The most common dosing error

The Institute for Safe Medication Practices (ISMP) issued an advisory in December 2024 [6] specifically on compounded GLP-1 vigilance. The most common error category is confusing units with milligrams — patients counting the markings on their syringe and treating each mark as a milligram of drug rather than a unit of volume. The 2024 FDA pharmacovigilance database recorded 520+ adverse event reports for compounded semaglutide and 480+ for compounded tirzepatide as of April 30, 2025, with documented cases of 5- to 20-fold overdose driven by unfamiliar measurement.

The mitigation: read your vial label carefully, verify the concentration with your pharmacy, and use this calculator (or your pharmacy's instructions) to convert a target mg dose into the correct volume in units before drawing.

Important disclaimer

This calculator is for educational purposes only and does not constitute medical advice or pharmacy dispensing instructions. Compounded GLP-1 medications are not FDA- approved. Decisions about reconstitution, dosing, and administration should follow your prescribing clinician's instructions and your compounding pharmacy's labeled directions. Patients with personal or family history of medullary thyroid carcinoma (MTC) or MEN 2 syndrome should NOT use semaglutide or tirzepatide because of the FDA boxed warning. Every coefficient and safety threshold in this tool was verified against primary sources by a research subagent on 2026-04-07.

Related tools and research

• GLP-1 unit converter — mg ↔ U-100 insulin syringe units (the building block of this calculator)
• Compounded GLP-1 reconstitution mixing guide — the same process in pictures, step by step
• PCAB accreditation: how to vet a compounded pharmacy
• FDA warning letters on compounded GLP-1s
• GLP-1 dose plotter — pharmacokinetic buildup curves
• GLP-1 pen leftover calculator — for patients on brand-name multi-dose pens (Ozempic, Saxenda) instead of compounded vials