Key takeaways
- Water (mL) = vial (mg) ÷ dose (mg) × units ÷ 100. A 10 mg vial, a 2.5 mg dose and 50 units per dose need 2 mL.
- Water changes the units, not the dose. A 10 mg vial holds 4 doses of 2.5 mg whether you add 1 mL or 3 mL.
- Pick a volume that lands on a printed mark, stays at 2 units or more and fits both the syringe and the vial.
- Bacteriostatic water contains 0.9% benzyl alcohol, so a vial can be entered more than once. It is not for use in newborns.
- Recalculate whenever the vial, the water or the syringe changes.
How to use the BAC water calculator
Most calculators ask how much water you added and tell you the units. This one works backwards. You say what you want the syringe to read, and it tells you how much bacteriostatic water produces that reading.
- Peptide in the vial (mg). Use the total printed on the label. For a vial with two or more peptides, use the peptide blend calculator.
- Dose per injection. Enter it in mcg or mg. The tool flags values that look like a unit slip, such as 250 mg where 250 mcg was meant.
- Syringe. Pick 0.3, 0.5 or 1 mL. The size sets the capacity and the spacing of the printed marks. Tick U-40 only if the barrel says U-40.
- Vial capacity. The most liquid your vial can hold. The default is 3 mL. Any answer above it triggers a warning.
- Your target. Choose units per dose (for example, each dose is 10, 25 or 50 units) or mcg per unit (for example, each unit is 50 mcg, which makes mental math easy).
- Read the result. The big number is the water to add. Below the result, the options table lists every 0.5 mL step up to your vial capacity. Highlighted rows put your dose exactly on a whole-unit mark.
What is bacteriostatic water?
Bacteriostatic water for injection (BAC water) is sterile water with a preservative added. On the Hospira label published on DailyMed, the preservative is benzyl alcohol at 0.9% (9 mg/mL) in the 30 mL plastic vial and 1.1% (11 mg/mL) in the glass vial. The pH is 5.7 (range 4.5 to 7.0). The label calls it a multiple-dose container and limits its use to diluting or dissolving drugs for intravenous, intramuscular or subcutaneous injection.
The preservative is the point. It holds back bacterial growth, so the vial can be punctured again for later draws. Sterile water for injection has nothing added. Its labels describe single-dose containers with no bacteriostat or antimicrobial agent and say to discard the unused portion.
| Feature | Bacteriostatic water | Sterile water |
|---|---|---|
| Preservative | Benzyl alcohol, 0.9% (9 mg/mL) in the plastic vial | None |
| Container | Multiple-dose vial | Single-dose vial |
| After first use | Can be entered again. CDC: date it, discard within 28 days unless the maker states otherwise | Discard the unused portion |
| Newborns | Not for use in neonates | No benzyl alcohol |
| Unopened storage | 20–25 °C (68–77 °F) | Room temperature, per label |
The BAC water formula
Every dose is a slice of the vial. If the vial holds a certain number of doses and you want each dose to take up a certain volume, the vial needs that many doses times that volume of water. Three steps get you there.
On one line for a U-100 syringe, and in the second form for a target mcg per unit:
Check: 10 ÷ 2.5 × 50 ÷ 100 = 2 mL.
Check: a 5 mg vial at 25 mcg per unit needs 5,000 ÷ 2,500 = 2 mL.
| Term | Meaning | Unit |
|---|---|---|
| Vial | Total peptide in the vial, from the label | mg |
| Dose | Amount in one injection | mcg or mg |
| Target units | What you want the syringe to read for one dose | units |
| mcg per unit | How much peptide each syringe unit carries | mcg |
| V | Volume of one dose | mL |
| Water | Bacteriostatic water to add to the vial | mL |
Why the water changes the units, not the dose
The powder in the vial is a fixed mass. Water only spreads it out. Add more water and each unit carries less peptide, so the same dose takes more units. The diagram mixes one 10 mg vial three ways and draws the same 2.5 mg dose from each.
So which volume is right? Any volume that passes four checks:
- The dose lands on a printed mark. A 1 mL syringe is usually marked every 2 units, a 0.5 mL syringe every unit, and some 0.3 mL syringes every half unit.
- The dose is at least 2 units. Below that, a small misreading is a large share of the dose.
- The dose fits the syringe. 75 units will not fit a 0.5 mL syringe.
- The water fits the vial, with room left for the powder and air.
Worked examples
Example 1: 10 mg vial, 2.5 mg dose
The vial holds 4 doses. For 25 units per dose, each dose is 0.25 mL, so add 4 × 0.25 = 1 mL. For 50 units per dose, add 2 mL. Note that 25 falls between the 2-unit marks of a 1 mL syringe but sits on a mark of a 0.5 mL syringe.
Example 2: 5 mg vial, 250 mcg dose, 10 units
The vial holds 20 doses of 0.1 mL each, so add 2 mL. The result is 2.5 mg/mL, or 25 mcg in every unit.
Example 3: 15 mg vial, 100 mcg per unit
Using the second formula, 15 × 1000 ÷ (100 × 100) = 1.5 mL. The vial is now 10 mg/mL, and every 2.5 mg is 25 units: 2.5 mg is 25 units, 5 mg is 50 units, 7.5 mg is 75 units.
Example 4: 60 mg vial, 2.5 mg dose, 10 units
The vial holds 24 doses of 0.1 mL, so add 2.4 mL. The mix is strong, 25 mg/mL, so each unit carries 250 mcg. A misread of one unit changes the dose by 10%, so a 0.3 mL syringe with finer marks helps.
Example 5: when the answer does not fit
A 30 mg vial with a 2.5 mg dose at 50 units needs 12 × 0.5 = 6 mL, twice what a 3 mL vial holds. The calculator flags it. Halve the target to 25 units and the answer drops to 3 mL, which fits.
How much BAC water for a 5, 10, 15, 20, 30 or 60 mg vial?
Each table shows the units per dose on a U-100 syringe after adding 1, 2 or 3 mL. The header gives mcg per unit for that volume. The doses are examples for the arithmetic, not suggestions. Above 100 units a dose needs more than one full 1 mL syringe. Below 2 units it is hard to measure.
5 mg vial
| If your dose is | 1 mL50 mcg/unit | 2 mL25 mcg/unit | 3 mL16.67 mcg/unit | Doses per vial |
|---|---|---|---|---|
| 100 mcg | 2 | 4 | 6 | 50 |
| 250 mcg | 5 | 10 | 15 | 20 |
| 500 mcg | 10 | 20 | 30 | 10 |
| 1 mg | 20 | 40 | 60 | 5 |
| 2.5 mg | 50 | 100 | 150over 1 mL | 2 |
10 mg vial
| If your dose is | 1 mL100 mcg/unit | 2 mL50 mcg/unit | 3 mL33.33 mcg/unit | Doses per vial |
|---|---|---|---|---|
| 250 mcg | 2.5 | 5 | 7.5 | 40 |
| 500 mcg | 5 | 10 | 15 | 20 |
| 1 mg | 10 | 20 | 30 | 10 |
| 2.5 mg | 25 | 50 | 75 | 4 |
| 5 mg | 50 | 100 | 150over 1 mL | 2 |
| 7.5 mg | 75 | 150over 1 mL | 225over 1 mL | 1.3 |
15 mg vial
| If your dose is | 1 mL150 mcg/unit | 2 mL75 mcg/unit | 3 mL50 mcg/unit | Doses per vial |
|---|---|---|---|---|
| 1 mg | 6.67 | 13.3 | 20 | 15 |
| 2.5 mg | 16.7 | 33.3 | 50 | 6 |
| 5 mg | 33.3 | 66.7 | 100 | 3 |
| 7.5 mg | 50 | 100 | 150over 1 mL | 2 |
| 10 mg | 66.7 | 133.3over 1 mL | 200over 1 mL | 1.5 |
| 12.5 mg | 83.3 | 166.7over 1 mL | 250over 1 mL | 1.2 |
20 mg vial
| If your dose is | 1 mL200 mcg/unit | 2 mL100 mcg/unit | 3 mL66.67 mcg/unit | Doses per vial |
|---|---|---|---|---|
| 2.5 mg | 12.5 | 25 | 37.5 | 8 |
| 5 mg | 25 | 50 | 75 | 4 |
| 7.5 mg | 37.5 | 75 | 112.5over 1 mL | 2.7 |
| 10 mg | 50 | 100 | 150over 1 mL | 2 |
| 12.5 mg | 62.5 | 125over 1 mL | 187.5over 1 mL | 1.6 |
| 15 mg | 75 | 150over 1 mL | 225over 1 mL | 1.3 |
30 mg vial
| If your dose is | 1 mL300 mcg/unit | 2 mL150 mcg/unit | 3 mL100 mcg/unit | Doses per vial |
|---|---|---|---|---|
| 2.5 mg | 8.33 | 16.7 | 25 | 12 |
| 5 mg | 16.7 | 33.3 | 50 | 6 |
| 7.5 mg | 25 | 50 | 75 | 4 |
| 10 mg | 33.3 | 66.7 | 100 | 3 |
| 12.5 mg | 41.7 | 83.3 | 125over 1 mL | 2.4 |
| 15 mg | 50 | 100 | 150over 1 mL | 2 |
60 mg vial
| If your dose is | 1 mL600 mcg/unit | 2 mL300 mcg/unit | 3 mL200 mcg/unit | Doses per vial |
|---|---|---|---|---|
| 2.5 mg | 4.17 | 8.33 | 12.5 | 24 |
| 5 mg | 8.33 | 16.7 | 25 | 12 |
| 7.5 mg | 12.5 | 25 | 37.5 | 8 |
| 10 mg | 16.7 | 33.3 | 50 | 6 |
| 12.5 mg | 20.8 | 41.7 | 62.5 | 4.8 |
| 15 mg | 25 | 50 | 75 | 4 |
Units = dose (mcg) × water (mL) ÷ (vial (mg) × 10). For a U-40 syringe, multiply each figure by 0.4.
How much BAC water for 10 mg or 15 mg tirzepatide?
The FDA-approved tirzepatide product, Zepbound, does not need bacteriostatic water. Its label describes a ready-to-use solution in single-dose pens and vials that each hold 0.5 mL, plus a multi-dose KwikPen. The Zepbound label starts at 2.5 mg once weekly for 4 weeks, then increases in 2.5 mg steps after at least 4 weeks, up to a maximum of 15 mg once weekly.
A vial of tirzepatide powder is not Zepbound, and that label does not cover how to mix it. The pharmacy or prescriber that supplied it should tell you the volume. The calculator checks the arithmetic. Because the label moves in 2.5 mg steps, a mix where 2.5 mg is a round number of units makes every step a round number too:
| Vial | Water for 5 mg/mL | 2.5 mg at 5 mg/mL | Water for 10 mg/mL | 2.5 mg at 10 mg/mL |
|---|---|---|---|---|
| 10 mg | 2 mL | 50 units | 1 mL | 25 units |
| 15 mg | 3 mL | 50 units | 1.5 mL | 25 units |
| 20 mg | 4 mLover 3 mL | 50 units | 2 mL | 25 units |
| 30 mg | 6 mLover 3 mL | 50 units | 3 mL | 25 units |
| 60 mg | 12 mLover 3 mL | 50 units | 6 mLover 3 mL | 25 units |
At 5 mg/mL each unit is 50 mcg; at 10 mg/mL each unit is 100 mcg. Flags assume a 3 mL vial.
So for a 10 mg vial, 1 mL gives 2.5 mg = 25 units, and 2 mL gives 50 units. For 15 mg, 1.5 mL gives 25 units and 3 mL gives 50 units. One catch: 25 and 75 units fall between the 2-unit lines of many 1 mL syringes, so the 5 mg/mL mix or a 0.5 mL syringe reads more cleanly. A 60 mg vial cannot be mixed to either strength inside a 3 mL vial, so it needs a stronger mix and more careful measuring. The tirzepatide calculator has presets for each vial size.
Retatrutide and other research peptides
Retatrutide is an investigational drug from Eli Lilly that acts on the GIP, GLP-1 and glucagon receptors. It is not FDA-approved. In a 2026 warning letter, the FDA called retatrutide products sold online unapproved new drugs, even when labelled for research use only. There is no approved label, so there is no labelled mixing volume.
In the phase 2 trial published in the New England Journal of Medicine, participants received retatrutide once weekly at 1, 4, 8 or 12 mg, and some arms started at 2 or 4 mg. Those were trial arms, not guidance for anyone outside the study.
The water math is the same for every powdered peptide, whether the label says retatrutide, BPC-157 or anything else. Use the table for your vial size, or the retatrutide calculator and BPC-157 calculator, which have presets.
How to mix a vial with BAC water, step by step
Follow the instructions that came with your product and your pharmacist's advice. This is the general sequence for a multi-dose vial.
- Work out the volume first. Use the calculator and write the number down.
- Wash your hands and work on a clean, dry surface.
- Wipe both rubber stoppers with a fresh alcohol swab and let them dry.
- Draw the water with a new sterile syringe. If the volume is more than the syringe holds, draw it in equal parts. With a 1 mL syringe, 2 mL is two full draws.
- Add the water slowly down the inside wall of the peptide vial instead of spraying it onto the powder.
- Swirl or roll gently until dissolved. Avoid shaking unless the label says otherwise.
- Inspect the solution. Do not use it if it is cloudy, discolored or has particles, unless the label says that is expected.
- Label the vial with the date, the water volume and the concentration. The peptide reconstitution calculator turns any dose into units for that mix.
How long does a vial last after adding BAC water?
Two clocks run at once. The first is the water itself. The Hospira label stores unopened bacteriostatic water at 20–25 °C (68–77 °F). Once any multi-dose vial has been punctured, CDC guidance is to date it and discard it within 28 days unless the manufacturer states another date.
The second is the mixed peptide. Stability depends on the product, so its label or the dispensing pharmacy sets the limit. Whichever date comes first wins. The doses per vial calculator shows whether you will finish the vial before then. If you pick a larger water volume, the vial does not last longer. It holds the same number of doses.
Common BAC water mistakes
Each of these maps to a warning in the calculator.
- More water than the vial holds. A big target on a big vial can ask for 6 mL or more. Lower the target units.
- A dose under 2 units. Very strong mixes make tiny draws, where one misread unit is a large error. Add more water.
- A dose that does not fit the syringe. 75 units needs a 1 mL syringe, not a 0.5 mL one.
- Using a U-40 syringe with U-100 math. A U-40 unit holds 2.5 times the volume. Every figure changes.
- Mixing up mg and mcg. 1 mg is 1,000 mcg. A slip here is a 1,000-fold error.
- Using sterile water for a vial you will enter many times. It has no preservative and is labelled for single use.
- Changing the water but not the units. 25 units from a 1 mL mix and 25 units from a 2 mL mix are different doses. The reverse peptide calculator shows what a given number of units contains.
Key terms
- Bacteriostatic water
- Sterile water with benzyl alcohol added as a preservative, supplied in multiple-dose vials.
- Sterile water for injection
- Sterile water with nothing added, supplied in single-dose containers.
- Benzyl alcohol
- The preservative in bacteriostatic water. It holds back bacterial growth between draws.
- Reconstitution
- Adding a liquid to a freeze-dried (lyophilized) powder to make a solution of known strength.
- Concentration
- Peptide per volume of solution, in mg/mL or mcg/mL.
- mcg per unit
- How much peptide one syringe unit carries. On a U-100 syringe it equals mg/mL × 10.
- Vial capacity
- The most liquid a vial can hold, including room for the powder.
Frequently asked questions
How much bacteriostatic water do I add to a 10 mg vial?
Any volume the vial can hold works, as long as you recalculate. 1 mL gives 10 mg/mL (100 mcg per unit), 2 mL gives 5 mg/mL (50 mcg per unit) and 3 mL gives about 3.33 mg/mL. For a 2.5 mg dose that is 25, 50 or 75 units. The reconstitution calculator gives units for any volume.
How much BAC water for 15 mg of tirzepatide?
It depends on how many units you want each dose to be. 1.5 mL makes 10 mg/mL, so 2.5 mg is 25 units. 3 mL makes 5 mg/mL, so 2.5 mg is 50 units. Zepbound itself is a ready-to-use solution and needs no water. See the tirzepatide calculator.
How much BAC water do you mix with peptides?
There is no single correct amount. Choose the volume that puts your dose on a printed syringe mark, keeps it at 2 units or more, fits your syringe and fits the vial. For most vial sizes, 1 to 3 mL gives readable unit counts, as the tables on this page show. The calculator above finds the exact volume for the number of units you want.
Does adding more BAC water make the peptide weaker?
It lowers the concentration, not the total amount. The vial still holds the same number of mg and the same number of doses. You simply draw more units for each dose. A 10 mg vial holds four 2.5 mg doses at 1 mL (25 units each) and at 3 mL (75 units each).
Can I use sterile water instead of bacteriostatic water?
Only if the product instructions allow it. Sterile water for injection has no preservative and is labelled for single use, with the unused portion discarded. Bacteriostatic water contains benzyl alcohol, which is why it is used for vials entered more than once.
Is bacteriostatic water safe for everyone?
No. The label says benzyl alcohol has been associated with toxicity in neonates and that solutions containing it should not be used in newborns. It is also labelled only for diluting or dissolving drugs, not for injecting on its own. Anyone with a known sensitivity to benzyl alcohol should ask a pharmacist which diluent to use.
Why does the calculator say the water will not fit in my vial?
Your target makes each dose a large volume, and the vial holds many doses. Water equals doses per vial times volume per dose. Pick fewer units per dose, or more mcg per unit, and the total falls. The doses per vial calculator shows how many doses the vial holds.
What if I already added the wrong amount of water?
The dose is still in the vial. Only the concentration changed. Enter the volume you actually added into the reconstitution calculator and use the new unit count. If you are not sure how much you added, ask your pharmacist before using the vial.
How many units is 2.5 mg after mixing?
Divide the vial mg by the water mL to get mg/mL, then divide 2.5 mg by that and multiply by 100. A 10 mg vial with 2 mL is 5 mg/mL, so 2.5 mg is 0.5 mL, or 50 units. The mg to units calculator does this for any mix.
Is this calculator a dosing recommendation?
No. It converts the numbers you enter into a water volume and syringe units. It does not tell you what dose to take, and the example doses in the tables are there only to show the arithmetic. Always follow your prescription label, your pharmacist and your clinician.
Sources
We check every formula and figure on this page against primary sources. See our editorial policy for how we review content.
- Bacteriostatic Water for Injection, USP (Hospira): prescribing information
- Sterile Water for Injection, USP, single-dose vial: prescribing information
- Zepbound (tirzepatide) injection: prescribing information
- Triple-hormone-receptor agonist retatrutide for obesity: a phase 2 trial (Jastreboff et al., N Engl J Med 389:514–526)
- Warning letter: Gram Peptides (retatrutide products as unapproved new drugs)
- Preventing unsafe injection practices
Reviewed by the PeptideConvert Editorial Team
Last reviewed . PeptideConvert tools do arithmetic only. They do not recommend a dose, and nothing on this site is medical advice. Always follow the label and your prescriber. Read the full medical disclaimer.