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 means 100 units per mL — FDA-approved U-100 insulin is labeled “100 units/mL” and “use only with a U-100 syringe.” So 100 units fill 1 mL, and 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:

• Labeled maximum dose [3, 4, 5]: nothing above the labeled maximum will compute at all — 7.2 mg a week for semaglutide (Wegovy's adult weight-reduction maximum), 15 mg a week for tirzepatide (Mounjaro or Zepbound). Any semaglutide dose above 2.4 mg also raises a warning: 2.4 mg is Wegovy's usual maintenance dose, the label allows more only for adults who have tolerated 2.4 mg for at least 4 weeks and need more weight reduction [3], and Ozempic's maximum is 2 mg [11]. Retatrutide is not FDA-approved; its ceiling is 12 mg, the highest weekly dose given in its phase 3 trial [12].
• Below 6 units (0.06 mL): that is under a fifth of the smallest (0.3 mL) syringe. In a syringe-accuracy study, errors over 5% became more likely below 20% of a syringe's labeled capacity (the study used 1 to 20 mL syringes) [10]. You get told, and the fix offered is less water and a stronger solution.
• Above 50 units (0.5 mL): that is a large volume for one under-the-skin injection (our own comfort limit, not a labeled one). Again the answer is less water.
• Above 1.0 mL: will not fit in one draw on any standard U-100 syringe. You get a warning to check what you entered.
• Figures that look wrong. A vial over 200 mg, or more than 50 mL of water, raises a warning to check what you typed.

Bacteriostatic water and beyond-use dating

Bacteriostatic water for injection (USP) [2] is a multiple-dose vial of sterile water with benzyl alcohol added as a preservative (0.9% in the common 30 mL vial), stored at 20 to 25°C. FDA recommends that vials containing multiple doses of sterile medication be used within 28 days after the first use, because the risk of contamination rises after that [8].

The same applies to the mixed GLP-1 vial. The compounding pharmacy prepares it under USP <797> [1] and labels it with a beyond-use date, but FDA tells patients to discard a compounded GLP-1 vial 28 days after first use, even if medication remains and even if the compounder's instructions say it can be used for longer [8]. Whichever date comes first is the one to keep.

Compounded GLP-1 legal status (April 2026)

FDA first found the tirzepatide shortage resolved on October 2, 2024, and confirmed it on December 19, 2024. 503A pharmacies were given until February 18, 2025 or a district-court ruling, whichever came later; the court denied the injunction on March 5, 2025, and FDA says that period has ended [7]. FDA found the semaglutide shortage resolved on February 21, 2025, with a 503A window until April 22, 2025 or a court ruling; the court ruled on April 24, 2025, and that period has also ended [7]. In its April 1, 2026 update, FDA says neither drug is on its shortage list, and a 503A pharmacy may not regularly compound essentially a copy of a commercially available drug unless a prescriber documents a change that makes a significant difference for an identified patient [7]. FDA also states that retatrutide cannot be used in compounding under federal law [8]. Verify your pharmacy's legal status before ordering.

The dosing error FDA has flagged

FDA has warned about dosing errors with compounded semaglutide [6]. In most of the reports it reviewed, patients drew up more than prescribed and gave themselves five to 20 times the intended dose, and FDA pointed to confusion between milliliters, milligrams and “units” as a possible contributor: in several reports, patients told to draw 5 units (0.05 mL) on a 1 mL U-100 syringe drew 50 units instead. As of May 31, 2026, FDA had received 990 adverse event reports associated with compounded semaglutide and more than 730 associated with compounded tirzepatide [8].

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 a personal or family history of medullary thyroid carcinoma (MTC), or with Multiple Endocrine Neoplasia syndrome type 2 (MEN 2), should NOT use semaglutide or tirzepatide because of the FDA boxed warning [3, 4, 5].

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