BAC Water Calculator: How Much Bacteriostatic Water to Add to Any Peptide Vial

Key takeaways

• The water you add sets the concentration; it never changes the dose. This calculator works backwards from the dose on your prescription and the syringe units you want that dose to fill: water in mL = vial mg × 1,000 × units ÷ (100 × dose in mcg).
• Your pharmacy label overrides any calculator, this one included. The Egrifta WR label is the proof: arithmetic asks for 1.45 mL, the label says 1.3 mL, and the label's 16-unit dose is the right one.
• Bacteriostatic water (0.9% benzyl alcohol) is for multi-dose vials drawn from over days or weeks. Sterile water has no preservative and is specified for single-dose vials injected at once, as on the Egrifta SV label. Use the diluent the label names.
• Compounded semaglutide and tirzepatide, and every Zepbound vial, are dispensed as solutions. There is nothing to add, and adding water dilutes the medication.
• We publish mixing examples only for products a US pharmacy can legally dispense with a label. For retatrutide and BPC-157 we explain why there is no such number.

Our reconstitution calculator runs forwards: water in, units out. This one runs backwards. Enter the milligrams in the vial, the prescribed dose in micrograms and the U-100 syringe units you want that dose to fill, and it returns the bacteriostatic water that produces that result, the concentration and the doses per vial. It is a checking tool: if your pharmacy label states a volume, that volume is the instruction.

The formula

On a U-100 syringe 100 units is 1 mL, so a dose that fills U units has a volume of U ÷ 100 mL. The concentration that puts the dose in that volume, and the water that dilutes the vial to it, follow:

• Target concentration (mcg/mL) = dose mcg × 100 ÷ units
• Water to add (mL) = vial mg × 1,000 × units ÷ (100 × dose mcg)
• Doses per vial = vial mg × 1,000 ÷ dose mcg

The default values are a worked example: a 6 mg vial of compounded sermorelin, a prescription written as 300 mcg, and a dose you want to read as 15 units. Water = 6 × 1,000 × 15 ÷ (100 × 300) = 3 mL, which gives 2,000 mcg/mL and 20 doses. That matches the volume one telehealth provider's published patient instructions give for its 6 mg vial (Defy) and the example on our sermorelin profile. The 300 mcg figure is an example prescription, not a recommendation: the published sermorelin trials gave 1 mg or 2 mg per injection, which from the same vial would be 50 or 100 units.

Water changes the concentration, never the dose

Add 3 mL to that 6 mg vial and 300 mcg is 15 units. Add 1.5 mL and the same 300 mcg is 7.5 units. The sermorelin in each dose is identical; only the volume carrying it changes. That is why "I take 15 units" means nothing without the concentration, and why changing the water between refills while keeping the same units changes the dose.

Pharmacies do not all make the same choice. The provider instructions above keep every vial at 2,000 mcg/mL by scaling the water, 3 mL for 6 mg, 4.5 mL for 9 mg and 7.5 mL for 15 mg, so 300 mcg is 15 units whichever vial arrives (Defy). Other pharmacies add 3 mL to every size, the convention in our sermorelin dosage chart; then the same 300 mcg is 15 units from a 6 mg vial, 10 units from a 9 mg vial and 6 units from a 15 mg vial. Both are correct. Confusing one with the other is a 2.5-fold error.

Vial size also decides how long a vial has to last. At 300 mcg a night the 6 mg vial lasts 20 nights, the 9 mg vial 30 and the 15 mg vial 50. A vial mixed with bacteriostatic water is good until the beyond-use date on its label and, by the CDC and USP convention for opened multi-dose vials, never more than 28 days after the first puncture (CDC). More water does not buy more time; doses left after that date are discarded. Storage details are in how to store peptides.

The label overrides any calculator: the Egrifta WR example

Egrifta WR, the brand tesamorelin approved in 2025, is the cleanest test because its label gives every number. The kit holds an 11.6 mg vial and a 30 mL multiple-dose vial of bacteriostatic water. The label says to add 1.3 mL, states that the result is 8 mg per mL, and sets the dose at 1.28 mg, which is 0.16 mL, or 16 units on the U-100 scale. One mixed vial supplies seven daily doses, stays at room temperature and is discarded 7 days after mixing (Egrifta WR label).

Now run the label through plain arithmetic. 11.6 mg in 1.3 mL looks like 8.9 mg/mL, which would make 1.28 mg about 14 units, not 16. Type 11.6 mg, 1,280 mcg and 16 units into the calculator and it asks for 1.45 mL, not 1.3. The label is right and the arithmetic is incomplete: 8 mg/mL is what 11.6 mg in about 1.45 mL of finished solution gives, and the likeliest reason is that the powder itself, 11.6 mg of tesamorelin plus 188.5 mg of hydroxypropyl betadex and mannitol, adds volume as it dissolves. The label does not explain the figure; the manufacturer measured it. Any freeze-dried vial can behave this way, which is the practical reason the label beats any calculation you do yourself. The tesamorelin profile covers the drug.

Bacteriostatic water or sterile water?

The two diluents look identical. The difference is the preservative, and the label decides which your vial needs.

Sources: Egrifta WR label, Egrifta SV label, Zepbound label, CDC.

Three rules follow. Use only the diluent the label names; both Egrifta labels say to use only the diluent provided, and they specify different ones for the same molecule (Egrifta SV label). A vial mixed with sterile water is a single-use container, and leftover contents are not kept for a later dose. And bacteriostatic water contains 0.9% benzyl alcohol, which is why it allows repeated withdrawals and why its label warns against use in newborns (Hospira). Distilled water, tap water and non-sterile saline are never acceptable; how to reconstitute peptides covers technique.

Practical limits when you check a volume

When the label gives a volume, follow it. When you are judging whether a prescription makes sense with your pharmacist, five limits matter:

• Readability. A dose of a few units is hard to measure; a 1 mL syringe is usually marked every 2 units, a 0.3 mL syringe every unit or half unit. See the units guide.
• Vial capacity. Small vials hold about 2 to 3 mL; the calculator flags anything over 3 mL.
• Injection volume. Most people prefer under 0.5 mL (50 units) per injection; 100 units is a full insulin syringe.
• Beyond-use date. A vial that holds 50 doses and expires in 28 days wastes the rest.
• Accuracy of the water. Measure it with a syringe: 0.9 mL instead of 1 mL makes every later dose about 11 percent stronger than the label assumes.

Vials you should not add water to

Compounded semaglutide and tirzepatide are normally dispensed as ready-made solutions in multiple-dose vials, and patients draw each dose in units straight from the vial. That format is where the reported errors happen: FDA's 2024 alert described patients drawing five to 20 times the intended dose, patients told to draw 5 units who injected 50, and prescribers who converted milligrams to units wrongly (FDA 2024). A poison-center series reported two 10-fold self-dosing errors from compounded vials (Lambson 2023), and in California poison-center records 33 of 36 exposures to compounded GLP-1 drugs were administration errors (Ho 2025). Zepbound vials are solutions at every strength, drawn as 0.5 mL (single-dose) or 0.6 mL (multi-dose) (Zepbound label). For these products the question is how to read the concentration, and adding water dilutes every dose that follows. Our compounded semaglutide guide shows what a safe label looks like.

Research vials are the other category. We do not publish a water volume or units for peptides no US pharmacy can legally dispense: there is no label to check the arithmetic against and no verified amount in the vial. FDA's 2026 warning letters to online peptide sellers cited reconstitution calculators and bundled bacteriostatic water as evidence that research-labeled vials were sold as drugs (ArentFox Schiff 2026). Our pages on 10 mg retatrutide and BPC-157 explain what is known instead, and the legal tracker lists what a pharmacy can make.

Getting a vial you can check

A vial worth calculating for comes with a prescription from a clinician licensed in your state, a named pharmacy, and a label stating the milligrams, diluent, volume, concentration and beyond-use date. The complete guide to peptide therapy walks through the process, getting a peptide prescription online covers telehealth, and peptide therapy cost lists October 2026 prices.

Bottom line

How much bacteriostatic water to add is the volume on your label. The water decides the concentration and the units, never the dose. Use this calculator to understand and check a label, remember that a measured concentration can differ from simple division, use sterile water only where the label names it, add nothing to vials that arrive as solutions, and treat any vial without a pharmacy label as a product no calculator can make safe.

Frequently asked questions

+−How much water do I mix with my peptides?

The volume printed on your pharmacy label. The water does not change how much peptide is in the vial or in your dose; it only sets the concentration, and therefore how many syringe units carry the dose. If you want to understand or check a label, the formula is water in mL = vial mg × 1,000 × units ÷ (100 × dose in mcg). For a 6 mg vial, a 300 mcg prescription and a 15-unit dose, that is 3 mL.

+−How much bac water for 5mg sermorelin?

Whatever your pharmacy specifies for that vial. The compounded sermorelin vials we could source from published patient instructions were 6, 9 and 15 mg; most 5 mg vials sold online are research-labeled products, for which we do not publish mixing figures. If a licensed pharmacy dispensed you a 5 mg vial, its label states the volume and the concentration, and the calculator on this page will show you how the dose in micrograms becomes syringe units.

+−Can I use sterile water instead of bacteriostatic water?

Only if the label says so. Sterile water has no preservative, so it is used for single-dose vials that are injected right after mixing: the Egrifta SV label, for example, has the 2 mg vial mixed with 0.5 mL of the supplied sterile water and the dose given immediately, with the rest discarded. A vial you will draw from for days or weeks needs bacteriostatic water, which contains 0.9% benzyl alcohol.

+−Does adding more bac water make the peptide weaker?

Each milliliter is weaker; the vial is not. A 6 mg vial still holds 6 mg whether you add 1.5 mL or 3 mL. What changes is how many units carry a dose: 300 mcg is 7.5 units at 4,000 mcg/mL and 15 units at 2,000 mcg/mL. The danger is keeping the old number of units after the concentration changes, which changes the dose.

+−How many units is 1 mL for peptides?

100 units on a standard U-100 insulin syringe, so 10 units is 0.1 mL and 50 units is 0.5 mL. Units measure volume only; how much peptide is in 10 units depends on the concentration of the vial.

+−I added the wrong amount of bac water. What now?

The peptide is still in the vial; only the concentration is different. Work out the new concentration (vial mg × 1,000 ÷ the water you actually added), recalculate the units for your prescribed dose, write the new concentration on the vial, and tell the dispensing pharmacy so the next label and your refills match. If you are not sure how much you added, ask the pharmacy before using the vial.

+−Do I need bac water for compounded semaglutide or tirzepatide?

Usually not. Compounded GLP-1 drugs are normally dispensed as ready-made solutions in multiple-dose vials, and brand Zepbound vials are solutions at every strength. Adding water to a solution dilutes it. For these products you only need to convert the dose on the label into syringe units using the concentration printed on the vial.

Sources

1. LabelTheratechnologies. EGRIFTA WR (tesamorelin) for injection, 11.6 mg per vial: full prescribing information, revised March 2025 (reconstitution with 1.3 mL bacteriostatic water, 8 mg/mL, 1.28 mg in 0.16 mL, 7-day use). DailyMed. (2025)
2. LabelTheratechnologies. EGRIFTA SV (tesamorelin) for injection, 2 mg per vial: full prescribing information, revised March 2024 (reconstitution with 0.5 mL sterile water for injection, 1.4 mg in 0.35 mL, use immediately). DailyMed. (2024)
3. LabelEli Lilly. Zepbound (tirzepatide) injection: prescribing information, revised August 2026 (single-dose vials 0.5 mL; multi-dose vials 2.4 mL, four doses of 0.6 mL, preserved with benzyl alcohol). DailyMed. (2026)
4. LabelHospira, Inc. Bacteriostatic Water for Injection, USP, multiple-dose vial (0.9% benzyl alcohol). Prescribing information (DailyMed).
5. GuidelineCDC. Injection safety: clinical safety guidance for healthcare providers (single-dose vials, multi-dose vials, 28-day rule). Reviewed March 2024. (2024)
6. SourceDefy Medical. Medication and injection instructions for sermorelin (6 mg vial with 3 mL, 9 mg with 4.5 mL and 15 mg with 7.5 mL of bacteriostatic water). Telehealth provider patient instructions, accessed October 10, 2026. (2026)
7. RegulatoryFDA. FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products. July 26, 2024. (2024)
8. StudyLambson JE, Flegal SC, Johnson AR. Administration errors of compounded semaglutide reported to a poison control center: case series. J Am Pharm Assoc 2023;63:1643-5. (2023)
9. StudyHo RY et al. Changes in glucagon-like peptide-1 receptor agonist (GLP-1 RA) exposures following recent demand for weight management: a retrospective review of California Poison Control System data. J Pharm Technol 2025;41:165-73. (2025)
10. NewsArentFox Schiff. RUO-ined: five peptide vendors learn that research use only is not a legal strategy (FDA warning letters of August 24, 2026). September 18, 2026. (2026)