Retatrutide Reconstitution Calculator

A retatrutide reconstitution calculator works backwards from the syringe. Pick how many units you want a dose to read, divide the vial's milligrams by the concentration that makes your dose fit, and the answer is the bacteriostatic (bac) water to add. For a 10 mg vial where 2 mg should read as 20 units on a U-100 insulin syringe, that is 1 mL. The calculator above does the division for any vial, dose and unit target, and the sections below show the math so you can check it by hand.

Retatrutide is still investigational. As of October 2026, Lilly has reported six positive Phase 3 trials, no approved version exists, and Lilly's latest release says retatrutide "cannot be legally sold or marketed for human use". We found no mixing or storage instructions from Lilly, so this page only does the arithmetic, and the 28-day limit further down is FDA's general rule for multi-dose vials, not a retatrutide stability result.

How the math works

Three steps get you from a vial to a number of mL.

1. Decide how many units you want each dose to read, for example 20 units.
2. On a U-100 syringe, 20 units is 0.2 mL, so your dose has to fit in 0.2 mL. For a 2 mg dose that means 2 mg in 0.2 mL, or 10 mg per mL.
3. Divide the vial's milligrams by that concentration. A 10 mg vial at 10 mg per mL needs 1 mL of water.

In one line: water in mL equals vial mg times (units divided by 100), divided by dose mg. A 30 mg vial where 4 mg should read as 20 units works out to 30 times 0.2 divided by 4, which is 1.5 mL, or 20 mg per mL. People search for "units of bac water", but you add water in mL. Units only describe what you draw afterward.

To check a mix you already made, run it forward. Concentration is vial mg divided by water mL. Volume is dose divided by concentration. Units are volume times 100.

Common mixes

The table gives the reading for three doses. In the Phase 3 trials, everyone started at 2 mg and stepped up every 4 weeks, to 4, 6, 9 and 12 mg depending on the arm (Lilly's TRIUMPH-1 release), so a mix that suits 2 mg has to be checked against the doses that come after it.

A 1 mL U-100 syringe holds 100 units, and smaller 0.5 mL and 0.3 mL syringes hold 50 and 30. Anything past the syringe's limit means two draws or a stronger mix. More water makes small doses easier to measure because each dose takes more space on the syringe. Less water keeps big doses inside one draw. Dose size and timing are for a doctor to decide. This page only converts the numbers.

How many doses fit before day 28?

FDA says multi-dose vials of sterile drugs should generally be used within 28 days of first use, and to discard the vial then even if liquid is left, "even if instructions from the compounder say the medication can be used for longer". Mixing punctures the stopper, so the clock starts the day you add water. At one dose a week, that is four doses, on days 0, 7, 14 and 21. The table assumes that schedule.

The 10 mg vial at 2 mg is the tight fit: its fifth dose would fall on day 28 itself, right at the edge of FDA's limit. Our 10 mg vial page works through that vial in full.

Our read: a bigger vial isn't a better deal if most of it gets thrown out on day 28. A 30 mg vial at 2 mg a week leaves 22 mg unused, and a vial that matches your dose wastes far less. We found no published stability data for mixed retatrutide, so FDA's 28 days is the best line available, and storage covers where to keep the vial in the meantime.

Check the numbers before you draw

The calculator can only divide. It can't know whether the vial holds what the label says. The TGA tested one vial sold as retatrutide and found no retatrutide in it, plus undeclared semaglutide at about 8 times the level in approved semaglutide products. Every number here assumes the label is true, and that is the one thing you can't check at home.

Mistakes in the units step are better documented. FDA's alert on compounded semaglutide describes patients who drew up five to 20 times the intended dose from multi-dose vials, mostly because they weren't sure how to measure a dose on the syringe, and prescribers who miscalculated when converting milligrams to units or milliliters. That is semaglutide, not retatrutide, but the arithmetic is identical. Write the water amount and the concentration in mg per mL on the vial the day you mix it, and recompute the units forward before the first draw. The units chart lists readings for other mixes, and how to reconstitute retatrutide covers the mixing steps themselves.

Vial size and cost

Compare vials by price per milligram, then by how much you'd use before day 28. The second table shows how much of a big vial can go unused. When you compare retatrutide for sale, check each listing's price per milligram and whether the batch has its own third-party test report. If you already know your water amount, the dose calculator turns it into units, and the weight loss calculator is a separate tool built on trial averages.