Interactive tool · Reconstitution
How much bacteriostatic water goes in the vial?
A semaglutide vial is labelled in milligrams. The number that matters once you are holding a syringe is milligrams per millilitre, and the only thing standing between the two is how much water went in. Work yours out below, or read it off the water chart.
What this tool is, and what it is not
This is arithmetic, not an instruction to mix anything. A US compounding pharmacy generally dispenses semaglutide already in solution; where it dispenses a lyophilised vial, it also supplies the diluent and the exact volume, and that instruction is the one that governs. Use this page to check that you have read it correctly and to understand what concentration it produces — never to substitute a figure of your own, and never as a reason to reconstitute a product that did not come from a pharmacy. Semaglutide is prescription-only. If the numbers here and the numbers on your paperwork disagree, the pharmacy is right and this page is not.
That mix gives you
5mg/mL
5 mg ÷ 1 mL
1 mL
total volume in the vial
Arithmetic only — this is not an instruction to reconstitute anything. Use the diluent volume your pharmacy specified; this tool is here so you can check that you read it right.
What that means when you draw
5units
on a U-100 syringe (0.05 mL)
20
doses in the vial, at this dose
How it is calculated. Reconstituting a vial fixes its concentration = total mg ÷ mL of bacteriostatic water, and the same relationship run backwards gives the water volume for a target concentration = total mg ÷ target mg/mL. A U-100 insulin syringe reads 100 units per mL, so units = (dose mg ÷ concentration) × 100, and the vial holds total mg ÷ dose mg doses. Nothing is stored and nothing leaves your browser.
Bacteriostatic water chart, by vial size
One division does all the work here: water (mL) = vial total (mg) ÷ target concentration (mg/mL). Find your vial down the left, the concentration you were told to reach across the top, and the cell is the volume of bacteriostatic water. Note what the table shows about pairs — a 10 mg vial made up to 2 mL and a 5 mg vial made up to 1 mL are the same 5 mg/mL strength, and every syringe mark on them is identical.
| Vial | for 1 mg/mL | for 2 mg/mL | for 2.5 mg/mL | for 5 mg/mL | for 10 mg/mL |
|---|---|---|---|---|---|
| 2 mg | 2 mL | 1 mL | 0.8 mL | 0.4 mL | 0.2 mL |
| 2.5 mg | 2.5 mL | 1.25 mL | 1 mL | 0.5 mL | 0.25 mL |
| 3 mg | 3 mL | 1.5 mL | 1.2 mL | 0.6 mL | 0.3 mL |
| 4 mg | 4 mL | 2 mL | 1.6 mL | 0.8 mL | 0.4 mL |
| 5 mg | 5 mL | 2.5 mL | 2 mL | 1 mL | 0.5 mL |
| 7.5 mg | 7.5 mL | 3.75 mL | 3 mL | 1.5 mL | 0.75 mL |
| 10 mg | 10 mL | 5 mL | 4 mL | 2 mL | 1 mL |
| 12.5 mg | 12.5 mL | 6.25 mL | 5 mL | 2.5 mL | 1.25 mL |
| 15 mg | 15 mL | 7.5 mL | 6 mL | 3 mL | 1.5 mL |
These volumes are what the arithmetic produces, not a recommendation about which concentration your vial should be. Practical limits apply that a calculator cannot see: a vial has a physical capacity, and adding 15 mL to a small vial is not possible whatever the division says.
Why this number is the one that goes wrong
In July 2024 the FDA issued an alert about dosing errors with compounded injectable semaglutide, after adverse-event reports that included patients seeking medical attention and being hospitalised. The agency was specific about the mechanism, and it is this page's subject: patients unfamiliar with drawing from a vial, instructions written in "units" whose volume varies with the concentration, and confusion between millilitres, milligrams and units.
The examples in the alert are worth reading in full because they are so ordinary. Patients directed to administer 5 units administered 50. A prescriber intending 0.25 mg — which is 5 units of a 5 mg/mL vial — wrote 25 units, and the patient took five times the intended dose and vomited severely. Another wrote 20 units instead of 2, across three patients. Reported overdose effects ran from nausea and vomiting through fainting and dehydration to acute pancreatitis and gallstones, and because semaglutide's half-life is around a week, an overdose is not something that clears by the evening.
Every one of those is a conversion error, not a judgement error. That is why the concentration belongs at the top of the page rather than buried in it, and why the honest answer to "how many units should I draw" always begins with a question about the vial. Once yours is fixed, the dose-to-units converter prints the whole ladder for it.
A worked example: the 5 mg vial
Take the most common setup. A 5 mg vial with 1 mL of bacteriostatic water is 5 mg ÷ 1 mL = 5 mg/mL. At that strength a 0.25 mg starting dose is 0.05 mL, which is the 5-unit mark on a U-100 insulin syringe — the exact figure the FDA alert uses in its own example. The vial holds twenty 0.25 mg doses, or about five weeks at a 1 mg maintenance dose. Here is the full ladder for it:
| Dose | Volume | U-100 syringe mark |
|---|---|---|
| 0.125 mg | 0.025 mL | 2.5 units |
| 0.25 mg | 0.05 mL | 5 units |
| 0.5 mg | 0.1 mL | 10 units |
| 0.75 mg | 0.15 mL | 15 units |
| 1 mg | 0.2 mL | 20 units |
| 1.25 mg | 0.25 mL | 25 units |
| 1.7 mg | 0.34 mL | 34 units |
| 2 mg | 0.4 mL | 40 units |
| 2.4 mg | 0.48 mL | 48 units |
Change the water to 2 mL and nothing about the prescription changes — but every number in the right-hand column doubles, because the same milligrams are now spread through twice the liquid. That is the whole reason a chart without a stated concentration is worthless.
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Common questions
- How much bacteriostatic water do I mix with a 5 mg semaglutide vial?
- It depends on the concentration your pharmacy is aiming for, and the pharmacy's instruction is the one to follow. The arithmetic is water mL = vial mg ÷ target mg/mL: 1 mL of bacteriostatic water in a 5 mg vial gives 5 mg/mL, 2 mL gives 2.5 mg/mL, and 5 mL gives 1 mg/mL. The vial size alone does not determine the answer.
- How much bacteriostatic water for a 10 mg vial?
- Same formula. A 10 mg vial with 1 mL of bacteriostatic water is 10 mg/mL; with 2 mL it is 5 mg/mL; with 4 mL it is 2.5 mg/mL. A 10 mg vial made up to 2 mL and a 5 mg vial made up to 1 mL are the same strength — the totals differ, the concentration does not.
- What is bacteriostatic water, and why not sterile water?
- Bacteriostatic Water for Injection is sterile water containing benzyl alcohol as a preservative, which lets a multi-dose vial be entered more than once without supporting bacterial growth. Plain Sterile Water for Injection has no preservative and is intended for single-dose use. Which diluent is correct for a given product is a pharmacy decision, not a preference — use what was dispensed with the vial.
- Does the water volume change my dose?
- It changes the volume you draw, not the milligrams your prescriber ordered. A more dilute vial means more units on the syringe for the same dose. What it can change is your risk of error: the FDA's 2024 alert on compounded semaglutide describes patients directed to administer 5 units who administered 50, and prescribers converting milligrams to units incorrectly, producing doses five to ten times higher than intended.
- Should I be reconstituting semaglutide myself?
- Usually there is nothing to reconstitute — a US compounding pharmacy generally dispenses semaglutide already in solution. Where a lyophilised vial is dispensed, the pharmacy supplies the diluent and the volume to use, and that instruction is the one that governs. This calculator is for checking that instruction, and for understanding what concentration it produces. It is not a substitute for one, and it is not a reason to mix a product bought outside a pharmacy.
Sources
- U.S. Food and Drug Administration. FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products (July 26, 2024).
- Novo Nordisk. WEGOVY (semaglutide) — FDA Prescribing Information, via DailyMed (NLM). Dosage and Administration.
Read next
This calculator is informational and not medical advice. It performs fixed arithmetic (concentration = mg ÷ mL; water mL = mg ÷ target mg/mL) on values you enter and makes no recommendation about whether, how much, or how to prepare any medication. Semaglutide is available by prescription after clinician review. Compounded vials are not FDA-approved finished products and vary by pharmacy; follow the reconstitution instruction dispensed with your vial and verify every number with your prescriber and pharmacy.