Peptide Vial Math

How to reconstitute Retatrutide

Add a measured volume of bacteriostatic water to the Retatrutide vial and swirl gently. 10 mg + 2 mL = 5 mg/mL, so 2 mg is 0.4 mL, or 40 units on a U-100 syringe.

Retatrutide vials come in 5, 10, 12, 15, 20, 24, 30, 40 mg, so what a unit holds depends on your vial.

By Peptide Vial Math Editorial Team Reviewed by the team on (how we review) Updated Formulas unit-tested (methodology)

Retatrutide reconstitution steps

  1. Read the vial label and confirm the total mg. This page covers 5, 10, 12, 15, 20, 24, 30, 40 mg.
  2. Choose the water volume using the matrix below and write down the resulting mg/mL and mg per unit.
  3. Warm the closed vial to room temperature; swab both stoppers with alcohol and let them dry.
  4. Add the bacteriostatic water slowly down the inside wall of the vial.
  5. Swirl gently until dissolved; don't shake. Inspect: clear, no particles.
  6. Label with total mg, mL added, mg/mL, mg per unit and the date.

The full procedure, with reasons for each step, is in the general reconstitution guide.

What concentration does each Retatrutide vial and water volume give?

What one U-100 unit holds for each Retatrutide vial size and water volume:

Retatrutide: mcg per U-100 unit (mg/mL in brackets) by vial size and water added
Vial1 mL2 mL2.5 mL3 mL
5 mg 50 (5)25 (2.5)20 (2)16.67 (1.67)
10 mg 100 (10)50 (5)40 (4)33.33 (3.33)
12 mg 120 (12)60 (6)48 (4.8)40 (4)
15 mg 150 (15)75 (7.5)60 (6)50 (5)
20 mg 200 (20)100 (10)80 (8)66.67 (6.67)
24 mg 240 (24)120 (12)96 (9.6)80 (8)
30 mg 300 (30)150 (15)120 (12)100 (10)
40 mg 400 (40)200 (20)160 (16)133.3 (13.33)

And the concentration each combination gives:

Retatrutide: concentration, mg/mL by vial size and water added
Vial1 mL2 mL2.5 mL3 mL
5 mg 52.521.67
10 mg 10543.33
12 mg 1264.84
15 mg 157.565
20 mg 201086.67
24 mg 24129.68
30 mg 30151210
40 mg 40201613.33

Retatrutide is sold in more vial sizes than almost any other compound (5 mg to 40 mg), so the same "units" reading means very different amounts from one vial to the next. A 40-unit draw is 2 mg from a 10 mg / 2 mL vial but 8 mg from a 40 mg / 2 mL vial. Always recompute when the vial size changes.

Retatrutide reconstitution calculator

Vial mg, water mL and amount give syringe units

mg

Total printed on the vial label.

mL

Diluent volume you add (1 mL = 100 units on a U-100 syringe).

Amount unit

The mass you want in one draw. 1 mg = 1,000 mcg.

Syringe barrel
Show U-40 and U-50 syringes

Result

40units

to draw on a U-100 syringe, 100-unit barrel

Vial: 10 mg with 2 mL diluent 10 mg +2 mL
Syringe filled to 40.0 of 100 units 0 10 20 30 40 50 60 70 80 90 100 U-100, 1 mL
Volume
0.4 mL
Concentration
5 mg/mL
Per unit
50 mcg
Draws per vial
5

10 mg + 2 mL = 5 mg/mL, so 2 mg is 0.4 mL, or 40 units on a U-100 syringe.

Arithmetic only. Not a dosing recommendation. Check every figure against your vial label and lab protocol.

Per-amount unit charts for each vial size: Retatrutide calculator (5 mg, 10 mg, 12 mg, 15 mg, 20 mg, 24 mg, 30 mg, 40 mg).

Frequently asked questions

How much bacteriostatic water do you add to a 10 mg Retatrutide vial?

There is no single correct volume; it sets the concentration. 10 mg in 1 mL is 10 mg/mL, in 2 mL is 5 mg/mL, in 3 mL is 3.33 mg/mL. With 2 mL, each U-100 unit holds 0.05 mg.

How do I reconstitute a 40 mg Retatrutide vial?

Same procedure, different numbers. 40 mg in 2 mL is 20 mg/mL, so each unit holds 0.2 mg, 4× more than a 10 mg vial with the same water. Units copied from a smaller vial will be wrong.

What is the Retatrutide reconstitution formula?

mg/mL = vial mg ÷ water mL; mL = amount ÷ mg/mL; U-100 units = mL × 100. Example: 10 mg + 2 mL = 5 mg/mL, so 2 mg is 0.4 mL, or 40 units on a U-100 syringe.

Can I change the water volume after reconstituting Retatrutide?

Adding more diluent later is possible arithmetically (the new concentration is the remaining mg ÷ the new total volume), but you must account for what has already been drawn. It is simpler and less error-prone to choose the volume once, before mixing, and label it.

How should the Retatrutide vial be stored after mixing?

Reconstitution changes how a vial has to be kept: a lyophilized powder and a solution have different temperature, light and time limits, and an opened multi-dose diluent vial has its own clock (CDC guidance: discard 28 days after first puncture unless the manufacturer gives another date). The peptide storage guide covers both states, and the bacteriostatic water guide covers the diluent. Label each vial with the amount, the water added, the concentration and the date.

Sources

  1. What to know about retatrutide — Eli Lilly and Company
  2. FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss — U.S. Food and Drug Administration
  3. A Study of Retatrutide (LY3437943) in Participants Who Have Obesity or Overweight (TRIUMPH-1), NCT05929066 — ClinicalTrials.gov
  4. Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial — N Engl J Med, 2023, PMID 37366315
  5. Handling and Storage Guidelines for Peptides — Bachem Knowledge Center
  6. Bacteriostatic Water for Injection, USP (Hospira) — prescribing label — DailyMed, U.S. National Library of Medicine

Links checked 2026-10-19.