How much bacteriostatic water do I add to a 30 mg Tirzepatide vial?

Any volume you can measure, because the water is not the dose — it is the dilution. 1 mL gives 30 mg/mL, so 1 mg sits at 3.3 units on a U-100 barrel; 2 mL gives 15 mg/mL, so 1 mg sits at 6.7 units on a U-100 barrel; 3 mL gives 10 mg/mL, so 1 mg sits at 10 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 30 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 Tirzepatide 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 30 mg vial end up at?

1 mL → 30 mg/mL, 2 mL → 15 mg/mL, 3 mL → 10 mg/mL, 5 mL → 6 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 — 30 mg in 2 mL becomes 10 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 30mg Tirzepatide? 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 — 30 mg is 30 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.

Approved

FDA-approved

Approved 13 May 2022 as Mounjaro for type 2 diabetes, and 8 November 2023 as Zepbound for chronic weight management; Zepbound added obstructive sleep apnea on 20 December 2024. Every approved presentation is a ready-to-use sterile solution — no FDA-approved tirzepatide product is a lyophilised powder requiring reconstitution. Powder vials come from compounders, and the FDA declared the tirzepatide shortage resolved on 19 December 2024, ending the shortage basis for compounding it.

What each water volume does to a 30 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	30 mg/mL	0.033 mL3.3 units on a U-100 barrel
2 mL	15 mg/mL	0.067 mL6.7 units on a U-100 barrel
3 mL	10 mg/mL	0.1 mL10 units on a U-100 barrel
5 mLMore than a typical 3 mL vial holds — check yours before adding this much.	6 mg/mL	0.167 mL16.7 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	1.5 mL	20 mg/mL
10 units0.1 mL on a U-100 barrel	3 mL	10 mg/mL
20 units0.2 mL on a U-100 barrel	6 mLWill not fit a 3 mL vial.	5 mg/mL
25 units0.25 mL on a U-100 barrel	7.5 mLWill not fit a 3 mL vial.	4 mg/mL

A 30 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 7.5 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 30 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 Tirzepatide 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 mL30 mg/mL	1.5 mg	3 mg	6 mg	7.5 mg	15 mg
2 mL15 mg/mL	0.75 mg	1.5 mg	3 mg	3.75 mg	7.5 mg
3 mL10 mg/mL	0.5 mg	1 mg	2 mg	2.5 mg	5 mg
5 mL6 mg/mL — over a 3 mL vial	0.3 mg	0.6 mg	1.2 mg	1.5 mg	3 mg

One unit is 0.3 mg at 1 mL of water and 0.1 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 30 mg vial is therefore a lot of peptide to resolve finely: small masses need the most water the vial will hold.

The Zepbound schedule, drawn from a 30 mg vial

Tirzepatide has an FDA label, so there is a schedule to resolve. These are the label’s own steps — reproduced and cited in full on the Tirzepatide page, with the interval each is held for — turned into what the barrel would read at each water volume this vial holds, and how many draws of each step 30 mg contains. The label describes a pen; a powder vial has no label, and the arithmetic below is the whole of what can be said about it.

Labelled step	Draws in 30 mg	at 1 mL	at 2 mL	at 3 mL
2.5 mgStarting dose	12	8.3 uBetween the marks on every standard barrel	16.7 uBetween the marks on every standard barrel	25 uPrinted on 0.3 and 0.5 mL barrels; between marks on a 1 mL
5 mgValid maintenance dose for weight reduction	6	16.7 uBetween the marks on every standard barrel	33.3 uBetween the marks on every standard barrel	50 uPrinted mark on every U-100 barrel
7.5 mgTitration step	4	25 uPrinted on 0.3 and 0.5 mL barrels; between marks on a 1 mL	50 uPrinted mark on every U-100 barrel	75 uPrinted on 0.3 and 0.5 mL barrels; between marks on a 1 mL
10 mgMaintenance; minimum maintenance dose for sleep apnea	3	33.3 uBetween the marks on every standard barrel	66.7 uBetween the marks on every standard barrel	100 uPrinted mark on every U-100 barrel
12.5 mgTitration step	2	41.7 uBetween the marks on every standard barrel	83.3 uBetween the marks on every standard barrel	125 uOver 100 units
15 mgMaximum recommended dose	2	50 uPrinted mark on every U-100 barrel	100 uPrinted mark on every U-100 barrel	150 uOver 100 units

Two things the table makes visible that a single water volume hides. First, no one concentration serves the whole ladder: a vial mixed so the first step reads comfortably puts the last step 6 times further up the barrel, and past 100 units it does not fit at all. Second, the draw count is a fact about the vial, not a duration — the label fixes how long each step is held, and that is on the compound page, not here.

Calculator inputs

Set up your vial

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

Syringe volume

Tirzepatide in the vial

Compounded vials are commonly 10–60 mg

Bacteriostatic water to add

Dose per injection

More optionsReverse calculation, schedule and cost

Enter the bacteriostatic water you added and get the volume to draw.

Injections per week

Discard afterdays

Vial priceoptional

Draw to

10units(0.1 mL)

Concentration: 10 mg/mL
Water added: 3 mL
Doses in vial: 30

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.

Three things the water volume does not change

How much peptide you have. A 30 mg vial contains 30 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 30 mg of peptide on its own displaces roughly 20 microlitres — 2 units on a U-100 barrel, about one or two printed marks spread across the whole vial, and 2% of a 1 mL fill or 0.7% 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 30 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 Tirzepatide in the vial has no such figure from a label, because every approved presentation is a ready-to-use pen and the label's in-use window describes the pen, not a powder someone else lyophilised. 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 Tirzepatide itself — its regulatory status, its half-life and the doses that appear in published studies — see the Tirzepatide 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
FDA labelFDA / DailyMed· 2026-08
MOUNJARO (tirzepatide) injection — FDA prescribing information
FDA labelFDA / DailyMed· 2026-08
ZEPBOUND (tirzepatide) injection — FDA prescribing information
Published studyNew England Journal of Medicine· 2025
Aronne LJ et al. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (SURMOUNT-5)
Published studyNew England Journal of Medicine· 2021
Frías JP et al. Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes (SURPASS-2)
GuidelineThe Pharmaceutical Journal· 2026-07
Switching between weight-loss medications