How to Use This Calculator
Choose the vial size (presets from 5 mg to 60 mg, or enter a custom amount), then enter the bacteriostatic water added in mL, the weekly dose in mg and the syringe size. Click Calculate to see the units and mL to draw, the concentration in mg/mL, how many weekly doses the vial holds and how many weeks it lasts. If the dose needs more than the selected syringe holds, the calculator says so.
Doses Studied in the Phase 2 Trial
In the Phase 2 obesity trial published in the New England Journal of Medicine in 2023, adults received retatrutide once a week at 1 mg, 4 mg, 8 mg or 12 mg for 48 weeks. The 4 mg, 8 mg and 12 mg groups did not start at their final dose: they began at 2 mg or 4 mg and stepped up every 4 weeks. These were trial doses given under medical supervision, and they are cited here as context, not as a schedule to follow. (Source: Jastreboff et al., NEJM 2023.)
Choosing the Water Volume and Syringe
Adding more water lowers the concentration, which makes each dose a larger, easier-to-read draw but can push it past what a 0.3 mL or 0.5 mL syringe holds. The BAC Water Calculator helps pick a water volume for a clean draw, and the Syringe Volume Calculator shows where that draw sits on each barrel size. To plan supply across several weeks, use the Peptide Vial Yield Calculator and the Peptide Cycle Cost Estimator. For semaglutide or tirzepatide, use the GLP-1 Calculator.
Who Should Use This
Researchers working with lyophilised retatrutide who need to reconstitute a vial and convert a weekly dose in mg into syringe units.
What This Calculator Does
The Retatrutide Dosage Calculator converts a vial size, the bacteriostatic water added and a weekly dose in mg into the draw on a U-100 insulin syringe. It shows the units and mL to draw, the concentration, how many weekly doses the vial holds and how many weeks the vial lasts at that dose.
Formula Used
Concentration (mg/mL) = Vial size (mg) ÷ BAC water (mL)Volume per dose (mL) = Weekly dose (mg) ÷ ConcentrationUnits to draw = Volume (mL) × 100 (U-100 syringe)Doses per vial = FLOOR(Vial size ÷ Weekly dose)Weeks per vial = Doses per vial (one dose a week)Example Calculation
Limitations
Retatrutide is investigational and not FDA-approved; this is a unit-conversion tool, not dosing guidance. It assumes the whole vial dissolves, a U-100 syringe and one dose a week, and it ignores dead space and draw loss.
Frequently Asked Questions
Is retatrutide FDA-approved?
No. Retatrutide has not been approved by the FDA for any use. It is an investigational medicine that is still being studied in clinical trials, so there is no official label, dosing schedule or prescribing information. This calculator only converts units and does not recommend a dose.
How many units is 2 mg of retatrutide?
It depends on the concentration of the vial. A 10 mg vial mixed with 2 mL of bacteriostatic water holds 5 mg per mL, so 2 mg is 0.4 mL, which is 40 units on a U-100 syringe. The same vial mixed with 1 mL holds 10 mg per mL, so 2 mg is 0.2 mL, or 20 units.
How long does a retatrutide vial last?
Divide the vial size by the weekly dose and round down: that is the number of full weekly doses, and so the number of weeks the vial lasts. A 20 mg vial at 4 mg a week gives 5 doses, or 5 weeks. A change in the weekly dose changes the answer, so recalculate at each dose.
Does the syringe size change the number of units?
No. On any U-100 insulin syringe, 100 units equal 1 mL, so 0.4 mL is 40 units on a 1 mL, 0.5 mL or 0.3 mL syringe. The smaller syringes simply hold less: a 40-unit draw does not fit in a 0.3 mL (30-unit) syringe, and the calculator warns when a dose is over capacity.