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)

The U-100 insulin syringe standard is the FDA-recognized medical specification: 100 units of insulin = 1 mL of liquid, so 1 unit = 0.01 mL. This is why every “5 unit” marking on a U-100 insulin syringe corresponds to 0.05 mL.

Why this calculator has hard limits

The calculator refuses to compute or strongly warns when the inputs would produce a clinically unsafe result. The guard rails are:

• FDA maximum weekly dose [3, 4, 5]: the calculator blocks any target dose above the FDA-approved maximum for the selected drug (semaglutide 2.4 mg/wk for Wegovy, tirzepatide 15 mg/wk for Mounjaro/Zepbound).
• Below 6 units (0.06 mL): drawing accuracy degrades meaningfully on a U-100 syringe when measuring less than 20% of capacity [10]. The calculator flags this and suggests adding less BAC water to raise concentration.
• Above 50 units (0.5 mL): exceeds the ergonomic upper limit for a single weekly subcutaneous injection. The calculator flags this and suggests adding less BAC water.
• Above 1.0 mL: no standard U-100 insulin syringe holds this in a single draw. Hard warning.
• Plausibility bounds: vial > 200 mg or BAC water > 50 mL trigger a verification prompt.

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-day in-use expiry at controlled room temperature, after which the vial should be discarded.

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 — narrative walkthrough with photos and step-by-step instructions
• 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