Quick answer: the amount of bacteriostatic (BAC) water is set by your prescriber, pharmacy or product label, not by a universal rule. The math is: concentration (mg/mL) = vial mg ÷ mL added, and U-100 units per mg = (mL added × 100) ÷ vial mg. For example, 30 mg + 3 mL = 10 mg/mL, which is 10 units per mg.
Branded Zepbound and Mounjaro are already liquid. Zepbound single-dose vials and pens and Mounjaro pens come as a ready-to-use solution. Never add water or anything else to them. See the Zepbound and Mounjaro prescribing information. This post is only about lyophilized (powder) vials prepared under a prescriber's or pharmacy's instructions.
Educational only, not medical advice. This article explains arithmetic. It does not recommend a diluent volume, a dose or any compound. Follow your prescriber's and pharmacist's instructions and the product label.
What's the formula?
From our pillar guide on how to reconstitute peptides:
- Concentration (mg/mL) = mg in the vial ÷ mL of liquid added
- Volume to draw (mL) = amount (mg) ÷ concentration
- U-100 syringe units = mL × 100
Shortcut: units per mg = (mL added × 100) ÷ vial mg.
The vial-by-water table
Arithmetic on hypothetical vials. None is a suggestion to use any volume or amount.
| Vial | Liquid added | Concentration | U-100 units per 1 mg | mg per 10 units |
|---|---|---|---|---|
| 30 mg | 1 mL | 30 mg/mL | 3.3 units | 3 mg |
| 30 mg | 1.5 mL | 20 mg/mL | 5 units | 2 mg |
| 30 mg | 2 mL | 15 mg/mL | 6.7 units | 1.5 mg |
| 30 mg | 2.5 mL | 12 mg/mL | 8.3 units | 1.2 mg |
| 30 mg | 3 mL | 10 mg/mL | 10 units | 1 mg |
| 30 mg | 4 mL | 7.5 mg/mL | 13.3 units | 0.75 mg |
| 30 mg | 5 mL | 6 mg/mL | 16.7 units | 0.6 mg |
| 30 mg | 6 mL | 5 mg/mL | 20 units | 0.5 mg |
| 40 mg | 1 mL | 40 mg/mL | 2.5 units | 4 mg |
| 40 mg | 1.5 mL | 26.67 mg/mL | 3.8 units | 2.67 mg |
| 40 mg | 2 mL | 20 mg/mL | 5 units | 2 mg |
| 40 mg | 2.5 mL | 16 mg/mL | 6.3 units | 1.6 mg |
| 40 mg | 3 mL | 13.33 mg/mL | 7.5 units | 1.33 mg |
| 40 mg | 4 mL | 10 mg/mL | 10 units | 1 mg |
| 40 mg | 5 mL | 8 mg/mL | 12.5 units | 0.8 mg |
| 40 mg | 6 mL | 6.67 mg/mL | 15 units | 0.67 mg |
| 60 mg | 1.5 mL | 40 mg/mL | 2.5 units | 4 mg |
| 60 mg | 2 mL | 30 mg/mL | 3.3 units | 3 mg |
| 60 mg | 2.5 mL | 24 mg/mL | 4.2 units | 2.4 mg |
| 60 mg | 3 mL | 20 mg/mL | 5 units | 2 mg |
| 60 mg | 4 mL | 15 mg/mL | 6.7 units | 1.5 mg |
| 60 mg | 5 mL | 12 mg/mL | 8.3 units | 1.2 mg |
| 60 mg | 6 mL | 10 mg/mL | 10 units | 1 mg |
Reconstitution calculator
Enter the numbers from your own vial, and pick the syringe in your hand. Check the U- number and size printed on the syringe box.
Concentration
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Draw volume
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Syringe units
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U-100 1 mL, holds 100
Draws in the vial
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Math only, not dosing advice. Follow your prescriber or label. Real vials give slightly less than the full-draw count: a little liquid stays in the vial and needle. Your last setup is saved in this browser only; Clear removes it.
Save your vial in Caliper and it remembers the concentration and use-by date you entered, alongside your dose log. The calculator above is a browser tool and is not part of the app.
Save my vial in CaliperPrefer a page for one vial size? Use the 30 mg reconstitution calculator, or the units-to-mg converter to check a draw.
What changes with a bigger vial?
- Concentrations get high fast. 60 mg in 2 mL is 30 mg/mL, so each 1-unit tick holds 0.3 mg. Small measuring errors matter more.
- More liquid may not fit. Check the vial's stated capacity before planning a large volume. Many vials can't hold 5 or 6 mL.
- The vial lasts longer than its use-by date may allow. Beyond-use dates after mixing come from the pharmacy or label, not from how much is left. See our storage guide.
What mistakes do people make?
- Mixing a branded pen or vial that's already liquid.
- Reusing an old unit count after mixing a new vial at a different volume. See new vial, different concentration.
- Confusing total mg on the label with mg/mL. See how to read a vial label.
- Rounding a repeating decimal (13.33 mg/mL) differently each time.
Tracking it in Caliper
Caliper logs each dose with one tap, tracks each vial in Supplies with its reconstitution or opened date and use-by date, and flags low stock. Caliper doesn't do dose math or tell you how much to draw. Use the site calculator above for the arithmetic and your prescriber's instructions for the amount.
FAQ
How much BAC water should I add to a 30 mg vial?
There's no universal amount; follow your prescriber's or pharmacy's instructions. The math: 30 mg ÷ mL added = mg/mL. For example, 3 mL gives 10 mg/mL, or 10 units per mg.
How many units per mg in a 60 mg vial with 3 mL?
60 ÷ 3 = 20 mg/mL. Units per mg = (3 × 100) ÷ 60 = 5 units per mg on a U-100 syringe.
Do I add BAC water to Zepbound or Mounjaro?
No. Branded Zepbound vials and pens and Mounjaro pens are already a liquid solution. Never add anything to them.
What's the formula for units per mg?
Units per mg = (mL added × 100) ÷ vial mg, for a U-100 syringe.
Sources
- Eli Lilly, Zepbound prescribing information
- Eli Lilly, Mounjaro prescribing information
- FDA, Dosing errors associated with compounded injectable semaglutide products
Keep the whole picture in one place
Caliper is a GLP-1 and peptide tracker. It logs doses, tracks vials with reconstitution and use-by dates, and keeps reminders and side effects on one timeline. It never recommends or calculates a dose.
Track your vials with Caliper →
For adults 18+.
Medical disclaimer: This article is for general educational purposes only and is not medical advice. It does not recommend any dose, compound, product, supplier or protocol. Always follow your prescriber's instructions and the product label, and talk to a pharmacist or healthcare provider about any question regarding your medication.