What is retatrutide?
Most people looking up retatrutide dosing first want to know what the compound actually is. Retatrutide, also known by its development code LY3437943, is an investigational peptide from Eli Lilly. It is often called a “triple agonist” because it was designed to act on three receptors at once: GIP, GLP-1 and glucagon. That sets it apart from semaglutide, which targets GLP-1 alone, and from tirzepatide, which targets GIP and GLP-1.
Online, the name usually refers to research-labeled vials of freeze-dried powder, commonly sold as 5 mg, 10 mg, 20 mg or 30 mg. Those vials are not the investigational product used in clinical trials. This page does not describe what retatrutide does in the body; it covers the arithmetic that links the powder, the bacteriostatic water and a U-100 syringe.
How much BAC water for a retatrutide vial?
Searches like “how much bac water to reconstitute 20mg retatrutide” or “how much bacteriostatic water to mix with retatrutide 10mg” assume there is one right answer. There isn’t. The water volume only sets the concentration: less water makes a stronger solution, so the same amount takes fewer units; more water makes a weaker solution, so it takes more units. The retatrutide BAC water chart below shows mcg per unit for the four common vial sizes across several water volumes.
| BAC water | 5 mg vial | 10 mg vial | 20 mg vial | 30 mg vial |
|---|---|---|---|---|
| 1 mL | 50 mcg/unit 5 mg/mL | 100 mcg/unit 10 mg/mL | 200 mcg/unit 20 mg/mL | 300 mcg/unit 30 mg/mL |
| 2 mL | 25 mcg/unit 2.5 mg/mL | 50 mcg/unit 5 mg/mL | 100 mcg/unit 10 mg/mL | 150 mcg/unit 15 mg/mL |
| 3 mL | 16.67 mcg/unit 1.667 mg/mL | 33.33 mcg/unit 3.333 mg/mL | 66.67 mcg/unit 6.667 mg/mL | 100 mcg/unit 10 mg/mL |
| 4 mL | 12.5 mcg/unit 1.25 mg/mL | 25 mcg/unit 2.5 mg/mL | 50 mcg/unit 5 mg/mL | 75 mcg/unit 7.5 mg/mL |
Each cell shows mcg per unit on a U-100 syringe (100 units = 1 mL) and the resulting concentration.
Read down a column to see how a retatrutide 10mg reconstitution, 20mg reconstitution or 30mg reconstitution changes as water is added. Read across a row to compare vial sizes at the same volume. Every cell is vial mg × 1,000 ÷ (water mL × 100). The BAC water to peptide ratio guide explains why the ratio, not the vial size alone, decides the syringe marks.
Worked example: retatrutide mixing math, step by step
Here is the retatrutide mixing calculation applied to one vial. For example, if the amount you already have is 1.5 mg and a 20 mg vial is mixed with 2 mL of BAC water:
The math: 20 mg ÷ 2 mL = 10 mg/mL (10,000 mcg/mL), which is 100 mcg per unit. 1.5 mg ÷ 10,000 mcg/mL = 0.15 mL = 15 units on a U-100 syringe.
The same amount in a smaller vial looks very different. For example, if the amount you already have is still 1.5 mg but the vial is 10 mg mixed with 2 mL:
The math: 10 mg ÷ 2 mL = 5 mg/mL (5,000 mcg/mL), which is 50 mcg per unit. 1.5 mg ÷ 5,000 mcg/mL = 0.3 mL = 30 units on a U-100 syringe.
Halving the vial size at the same water volume halves the concentration, so the same amount needs twice as many units. That is why two people quoting “units” for retatrutide can mean completely different amounts. Units measure liquid, not drug: on any U-100 syringe, 100 units equal 1 mL, and what those units contain depends entirely on the concentration you created.
Retatrutide dosing and dosing schedule searches: what this page does
“Retatrutide dosing” and “retatrutide dosing schedule” are among the most searched phrases for this compound. This page answers only the math half of that question. The calculator takes three inputs, the mg printed on the vial, the mL of BAC water and an amount you enter, and returns a concentration, a volume in mL and a U-100 unit mark.
It does not choose the amount, and it does not produce a schedule. Retatrutide is not an approved drug, so there is no approved label for it, and any schedule is a matter for trial protocols and a qualified professional, not a calculator. What the page can do is make sure the conversion is right. The classic error is mixing mg and mcg: a retatrutide 5mg vial holds 5,000 mcg, because 1 mg = 1,000 mcg. If you enter an amount in mcg against a vial in mg without converting, the answer is off by a factor of a thousand. The mg to mcg conversion guide covers this in detail.
Retatrutide half-life and what it means for the math
Retatrutide was designed to be long-acting, and published trials have studied it as a once-weekly injection. That describes the investigational drug in a clinical setting; it is not a schedule, and this page makes no calculation based on half-life. For reconstitution math, half-life does not matter at all: the concentration is set only by the vial mg and the water mL.
What does matter over time is storage. Some suppliers specify refrigerating the vial once reconstituted and adding the water slowly down the inside wall, then swirling gently rather than shaking. Those are statements from product labeling, not instructions from this site. A punctured multi-dose vial of BAC water is commonly dated and discarded within 28 days unless the manufacturer states otherwise, which is worth remembering with larger vials that take longer to use up. See how long BAC water lasts for the details.
Is retatrutide FDA-approved?
No. Retatrutide is an investigational compound from Eli Lilly and, at the time of writing, is not FDA-approved for any use. Research-labeled vials sold online are not the clinical-trial product, and a correct calculation cannot confirm their identity, purity, sterility or content. The calculator assumes the vial holds exactly what its label states. To compare other compound-labeled calculators, browse the peptide library.
Retatrutide FAQ
How much BAC water do you add to a 10 mg retatrutide vial?
There is no single required volume. The water only sets the concentration. The chart on this page shows mcg per unit for several volumes so you can see how each choice changes the syringe marks; the product labeling decides the diluent and volume.
Does this page give a retatrutide dosing schedule?
No. It converts an amount you already have into mL and U-100 syringe units for your vial and water volume. It does not recommend a dose, schedule or frequency. Retatrutide has no approved label.
Why do 20 mg and 30 mg retatrutide vials give different units for the same amount?
With the same water volume, a larger vial makes a more concentrated solution, so the same amount fits in fewer units. The chart shows the vial sizes side by side.
What is the half-life of retatrutide?
Retatrutide was designed to be long-acting and has been studied as a once-weekly injection in trials. Half-life does not affect reconstitution math, which depends only on vial mg and water mL.
Is retatrutide FDA-approved?
No. At the time of writing it is an investigational Eli Lilly compound. Research-labeled vials sold online are not the product studied in clinical trials.