How much bacteriostatic water do I add to a 15 mg Retatrutide vial?

Any volume you can measure, because the water is not the dose — it is the dilution. 1 mL gives 15 mg/mL, so 1 mg sits at 6.7 units on a U-100 barrel; 2 mL gives 7.5 mg/mL, so 1 mg sits at 13.3 units on a U-100 barrel; 3 mL gives 5 mg/mL, so 1 mg sits at 20 units on a U-100 barrel. Pick whichever of those puts the amount you intend to draw on a mark you can read without squinting.

Is there a standard water volume for a 15 mg vial?

No. No manufacturer instruction, pharmacopoeial monograph or regulator specifies one, because the vial is a powder of a fixed mass and the diluent volume is a choice made by whoever reconstitutes it. Approved lyophilised products come with a diluent volume printed on the label; a Retatrutide vial bought as a powder does not, so the volume is yours to choose and the arithmetic above is the whole of the decision.

What concentration does a 15 mg vial end up at?

1 mL → 15 mg/mL, 2 mL → 7.5 mg/mL, 3 mL → 5 mg/mL, 5 mL → 3 mg/mL. Divide the milligrams in the vial by the millilitres of water you added — that single division is the only thing separating a mass on a label from a mark on a syringe.

Can I add more water afterwards if I picked wrong?

You can add more liquid to a vial and the total peptide stays the same, so the concentration simply falls — 15 mg in 2 mL becomes 5 mg/mL if you take it to 3 mL. What you cannot do is go the other way: nothing removes water once it is in. That asymmetry is the argument for working out the reading you want before you pierce the stopper, which is what the second table above does.

Bacteriostatic or sterile water?

Bacteriostatic Water for Injection is water with 0.9% benzyl alcohol added as a preservative, which is what lets a vial be entered more than once. Sterile Water for Injection has no preservative and is single-use. Both are FDA-approved diluents with their own labelling; neither is specific to peptides, and the benzyl alcohol carries its own contraindications — it must not be used in newborns.

How Much Bac Water for 15mg Retatrutide? Bacteriostatic Water Chart

There is no correct number, and any page that hands you one is choosing on your behalf. The water does not change what is in the vial — 15 mg is 15 mg whether you add 1 mL or 5 mL. All it sets is the concentration, and through that, how many marks on the barrel one milligram occupies. The right volume is the one that puts the amount you intend to draw somewhere you can read it.

Written by Kashif Khan, who builds and maintains this site and is not a clinician. Every figure links to its primary source.

Not approved

Investigational — not approved in any country

Retatrutide has reported positive topline results from multiple phase 3 TRIUMPH trials, but it holds no approval from the FDA, EMA, MHRA, TGA or Health Canada. Lilly says it plans an FDA submission in the first quarter of 2027; that is a future application, not an approval. The only lawful human exposure remains a clinical trial or expanded access. In August 2026 the FDA named retatrutide, under the street label “GLP-3R”, in a warning letter as an unapproved new drug.

No approved human dose exists for this compound in any country. The figures below are the doses used in named published studies, reproduced for reference only. Doses given to rats or mice do not convert to a human dose. The FDA has stated that labelling a product “research use only” or “not for human consumption” does not change its legal status when the seller's own marketing shows it is intended for people.

Anti-doping: Prohibited at all times. Falls under WADA class S0 (non-approved substances) by definition, because no regulator has approved it for human therapeutic use.

What each water volume does to a 15 mg vial

Everything here is stated per milligram of peptide, because a milligram is a quantity that is true of any vial and commits you to nothing. On a U-100 syringe, 100 units is 1 mL — so the last column is just the middle column turned into barrel markings.

Water added	Concentration	1 mg draws as
1 mL	15 mg/mL	0.067 mL6.7 units on a U-100 barrel
2 mL	7.5 mg/mL	0.133 mL13.3 units on a U-100 barrel
3 mL	5 mg/mL	0.2 mL20 units on a U-100 barrel
5 mLMore than a typical 3 mL vial holds — check yours before adding this much.	3 mg/mL	0.333 mL33.3 units on a U-100 barrel

Or work backwards from the reading you want

Most people have the question the other way round: they know what they want the barrel to say and want to know what to pour in. Decide how many units one milligram should occupy, and the water volume follows.

If 1 mg should read	Add this much water	Which is
5 units0.05 mL on a U-100 barrel	0.75 mL	20 mg/mL
10 units0.1 mL on a U-100 barrel	1.5 mL	10 mg/mL
20 units0.2 mL on a U-100 barrel	3 mL	5 mg/mL
25 units0.25 mL on a U-100 barrel	3.75 mLWill not fit a 3 mL vial.	4 mg/mL

A 15 mg vial is a lot of peptide in a small container, so fine readings cost volume you do not have. Spreading 1 mg over 25 units would take 3.75 mL, well past what a standard 3 mL vial holds. Either accept a coarser reading, or use a syringe that reads in finer steps.

What each mark is worth in a 15 mg vial

The two tables above work per milligram. This one works per mark, because the mark is what you see while the plunger moves: for each water volume, the milligrams of Retatrutide sitting at 5, 10, 20, 25 and 50 units on a U-100 barrel. Read across for one vial you have already mixed; read down to see how the same mark changes meaning with the water. It is a statement about the container, not a suggestion of what to draw.

Water added	5 units	10 units	20 units	25 units	50 units
1 mL15 mg/mL	0.75 mg	1.5 mg	3 mg	3.75 mg	7.5 mg
2 mL7.5 mg/mL	0.375 mg	0.75 mg	1.5 mg	1.875 mg	3.75 mg
3 mL5 mg/mL	0.25 mg	0.5 mg	1 mg	1.25 mg	2.5 mg
5 mL3 mg/mL — over a 3 mL vial	0.15 mg	0.3 mg	0.6 mg	0.75 mg	1.5 mg

One unit is 0.15 mg at 1 mL of water and 0.05 mg at 3 mL — the resolution of the whole exercise. On a 1 mL barrel printed in 2-unit steps the smallest mass you can actually stop on is twice that. A 15 mg vial is therefore small enough that a 0.3 or 0.5 mL barrel, printed in single units, resolves it better than a 1 mL barrel does.

Calculator inputs

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Enter the label, dilution and dose. The result updates as you type.

Syringe volume

Retatrutide in the vial

Vials sold online are commonly 5–30 mg

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More optionsReverse calculation, schedule and cost

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13units(0.13 mL)

Concentration: 7.5 mg/mL
Water added: 2 mL
Doses in vial: 15

How the concentration is calculated

This divides the peptide by the water you add. The powder occupies a little volume of its own, which is negligible on a typical vial but not on one with a heavy bulking agent. When a product label states a concentration, use the label’s number.

Drawing to the nearest printed mark delivers 975 mcg rather than exactly 1 mg.

This syringe reads in 1-unit steps, so the closest drawable mark is 2.5% away from the target dose.

Three things the water volume does not change

How much peptide you have. A 15 mg vial contains 15 mg of powder-weight peptide at every dilution on this page. Water adds volume, not mass.
How many doses it holds. That is set by the size of each draw in milligrams, not by the concentration. Halving the concentration doubles the volume of every draw and leaves the dose count identical.
Whether the product is what the label says. Arithmetic is exact; the contents of an unlabelled vial are not. Nothing on this page can tell you what is actually in one.

Does the powder itself take up room?

Slightly — a lyophilised cake displaces a small volume of its own, so the finished solution is fractionally more than the water you injected, and the true concentration is fractionally lower than the division suggests. For milligram quantities of peptide the effect is small enough to ignore next to the error in reading a syringe barrel, but it is real and it is measurable: displacement volumes have been published for reconstituted powder injections, and the FDA’s guidance on allowable excess volume exists because vial contents and labelled fill are not the same thing. Every figure on this page treats the diluent volume as the final volume, which is the same convention approved labels use.

Sized for this vial: measured displacement for lyophilised solids runs about 0.66 to 0.70 mL per gram, so 15 mg of peptide on its own displaces roughly 10 microlitres — 1 units on a U-100 barrel, about one or two printed marks spread across the whole vial, and 1% of a 1 mL fill or 0.3% of a 3 mL one. That is the peptide alone. A vendor vial carrying mannitol or another bulking agent displaces more, in proportion to its total solids, and the label rarely states them — which is the one input this arithmetic cannot supply.

How long a 15 mg vial lasts once the water is in

Two clocks start, and they are routinely confused. The bacteriostatic water bottle you drew from is a multiple-dose container, and USP <797> limits a punctured multiple-dose container to 28 days unless its manufacturer says otherwise — the water label itself carries no discard period at all. That is a rule about the water bottle. The reconstituted Retatrutide in the vial has no such figure from anywhere, because there is no approved product and therefore no label with a beyond-use date. The water label says only: do not store reconstituted solutions unless the solute’s manufacturer directs otherwise. The calculator above will show whether the vial runs out before whatever discard date you choose to apply; it cannot supply the date. The seven approved products that do publish one range from “use immediately” to 60 days.

Reading it off the barrel

Every human insulin syringe is U-100, so 0.3 mL, 0.5 mL and 1 mL barrels all read 100 units to the millilitre and the numbers above hold on any of them. What differs is how finely they are printed — 1 mL barrels are commonly marked in 2-unit steps, which is where small draws go wrong. A U-40 veterinary syringe is a different scale entirely and reading one as if it were U-100 produces a 2.5-fold error.

More on this in insulin syringe units, what is actually in bacteriostatic water and how to reconstitute a peptide. For Retatrutide itself — its regulatory status, its half-life and the doses that appear in published studies — see the Retatrutide page.

Common questions

Sources

Every number on this page comes from one of the documents below. Where no reliable source exists, the page says so instead of filling the gap.

FDA labelFDA / DailyMed
Bacteriostatic Water for Injection, USP — FDA prescribing information (NDA 018802)
FDA labelFDA / DailyMed
Sterile Water for Injection, USP — FDA prescribing information
GuidelineUnited States Pharmacopeia· 2023
USP General Chapter <797> Pharmaceutical Compounding — Sterile Preparations (2023 revision)
Published studyBMJ Paediatrics Open· 2023
Zheng L-Y et al. Displacement volume of reconstituted powder injections and its effect on dosing accuracy
Published studyNew England Journal of Medicine· 2023
Jastreboff AM et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity — a Phase 2 Trial
Published studyThe Lancet· 2022
Urva S et al. LY3437943, a novel triple GIP/GLP-1/glucagon receptor agonist — phase 1
RegulatorFDA· 2026-08-24
Warning Letter — Peak Performance Peptides (research-use-only labelling does not exempt an unapproved new drug)
ReferenceU.S. National Library of Medicine
ClinicalTrials.gov trial registry
ReferenceClinicalTrials.gov· 2026-06-03
NCT05929066 — TRIUMPH-1 phase 3 trial of retatrutide in obesity or overweight
ReferenceEli Lilly and Company· 2026-05-21
TRIUMPH-1 topline phase 3 results for retatrutide in obesity
ReferenceEli Lilly and Company· 2026-07-23
TRIUMPH-2 and TRIUMPH-3 topline phase 3 results for retatrutide
RegulatorWorld Anti-Doping Agency· 2026
WADA 2026 Prohibited List
ReferenceNIH / National Center for Biotechnology Information
PubChem compound records (molecular weights and formulae)