Semaglutide Dosing Calculator
A vial of compounded semaglutide does not tell you how much liquid to draw. It tells you how many milligrams of powder it holds, and your prescribed dose is a much smaller number of milligrams. Between those two numbers sits the arithmetic this calculator does: how strong the mixed solution is, what volume carries your dose, and where that volume falls on a U-100 insulin syringe. Enter the vial strength, the water volume and your prescribed dose, and it returns all three.
Educational arithmetic only. This calculator does not prescribe a dose, choose a product, or replace your prescriber. Brand-name semaglutide pens are prefilled and need none of this math; this tool is for vials, where the measuring is done by the patient. Compounded semaglutide is not FDA approved. If any number here disagrees with your prescription or pharmacy label, follow the prescription and ask your pharmacist.
Quick Answer
Quick answer: A 5 mg vial mixed with 2 mL of water gives a concentration of 2.50 mg/mL, so a prescribed 0.25 mg dose means drawing 0.100 mL, which reads as 10.0 units on a U-100 insulin syringe. The same three divisions answer every vial: strength divided by water gives concentration, dose divided by concentration gives volume, volume times 100 gives units.
Key takeaways
- Milligrams, millilitres and units are three different things. The dose is in milligrams; the syringe reads in units; the volume in millilitres connects them.
- On a U-100 syringe, 100 units equals 1 mL, so units are always volume times 100. A "10 units" instruction is 0.10 mL, never 10 mg.
- Changing the water volume changes every draw from that vial. Recompute whenever the vial or the water volume changes; never carry units over from an old vial.
- Brand-name pens need none of this: the dose is set by the device. This math exists only because a vial hands the measuring job to the patient.
- Quick Answer
- How to use this calculator
- The math, in three divisions
- Worked example, step by step
- Draw volumes across the label titration ladder
- What this tool is not
- Safety: why unit confusion hurts people
- What competing calculators miss
- Frequently asked questions
- Add This Calculator to Your Website
- References
How to use this calculator
Enter the three numbers exactly as they appear on your vial and prescription. Vial strength is the total milligrams printed on the vial label, not the concentration and not the dose. Reconstitution volume is the millilitres of bacteriostatic water added to that vial. Desired dose is the milligrams your prescriber set for one injection. Press Calculate and read the syringe units first, since that is the number you act on, then check the concentration and volume underneath as a sanity check.
The inputs are pre-filled with the worked example from this article: a 5 mg vial, 2 mL of water, and a 0.25 mg dose. If any input is missing, zero or negative, the calculator asks for valid values instead of producing a number, because a draw volume computed from a missing input would be dangerous to show. Reset restores the worked example and hides the result.
The math, in three divisions
Reconstitution math is dilution math, and it runs in one direction only.
Step 1: concentration. Concentration in mg/mL equals vial strength in mg divided by water volume in mL. A 5 mg vial plus 2 mL gives 2.50 mg/mL. The same vial with 1 mL would give 5.00 mg/mL, twice as strong, and every draw from it would be half the volume.
Step 2: volume to inject. Volume in mL equals dose in mg divided by concentration in mg/mL. A 0.25 mg dose from a 2.50 mg/mL solution needs 0.100 mL. Notice the units cancel: mg divided by mg/mL leaves mL, which is exactly what a syringe measures.
Step 3: syringe units. Units equal volume in mL times 100, because a U-100 insulin syringe holds 100 units per millilitre. So 0.100 mL reads as 10.0 units. Working backwards checks the answer: 10 units is 0.10 mL, times 2.50 mg/mL gives 0.25 mg, the prescribed dose.
Worked example, step by step
A vial holds 5 mg of semaglutide. It is mixed with 2 mL of bacteriostatic water. The prescribed dose is 0.25 mg. How far should the syringe be drawn?
Step 1: 5 divided by 2 gives a concentration of 2.50 mg/mL.
Step 2: 0.25 divided by 2.50 gives 0.100 mL to inject.
Step 3: 0.100 times 100 gives 10.0 units on a U-100 syringe.
Enter 5, 2 and 0.25 in the calculator and you get exactly those three numbers. Change the water to 1 mL and the same dose becomes 5.0 units, because the solution is twice as strong. That sensitivity is the whole point of recomputing per vial.
Draw volumes across the label titration ladder
The FDA-approved Wegovy injection schedule climbs in four-week steps: 0.25 mg for weeks 1 to 4, 0.5 mg for weeks 5 to 8, 1.0 mg for weeks 9 to 12, 1.7 mg for weeks 13 to 16, then 2.4 mg as maintenance from week 17 onward. The schedule itself is a prescriber decision, and our semaglutide dosage calculator tracks it week by week across the three brand products, including Ozempic, which tops out at 2.0 mg rather than 2.4 mg. The tables below translate that ladder into syringe draws for the two most common vial mixes, using this calculator's own formulas. They are translations of a prescribed dose, not prescriptions: your prescriber may hold you at a lower step or delay an increase, and that decision overrides any table.
| Prescribed dose | 5 mg vial + 2 mL (2.50 mg/mL) | 10 mg vial + 2 mL (5.00 mg/mL) |
|---|---|---|
| 0.25 mg | 0.100 mL, 10.0 units | 0.050 mL, 5.0 units |
| 0.5 mg | 0.200 mL, 20.0 units | 0.100 mL, 10.0 units |
| 1.0 mg | 0.400 mL, 40.0 units | 0.200 mL, 20.0 units |
| 1.7 mg | 0.680 mL, 68.0 units | 0.340 mL, 34.0 units |
| 2.4 mg | 0.960 mL, 96.0 units | 0.480 mL, 48.0 units |
Read the danger out of the table too: the same 0.25 mg dose is 10 units from one vial and 5 units from another. Anyone who memorises "10 units" and carries it to a new vial can double or halve their dose without noticing. Always recompute from the vial in hand.
What this tool is not
This calculator answers a vial question, and vial questions are only one corner of semaglutide dosing. It does not tell you which product to use, what dose to start on, when to increase, or what to do after a missed dose. Those are prescriber decisions from the FDA-approved label schedules, which differ by product: Wegovy injection climbs to 2.4 mg, Ozempic to 2.0 mg, and the oral tablet follows a different daily schedule entirely. If your question is "what dose does the label call for in my treatment week", use the dosage schedule calculator linked above, not this one.
There is also a second, reversed question this tool does not answer: given a dose and a comfortable draw volume, how much water should go into the vial? That is the approach of our peptide reconstitution calculator, which solves for water instead of solving for the draw. The two tools are complements, not rivals: water-first when the water is already fixed, dose-first when you are still deciding how much water to add. For general background on how prescribed doses are expressed and converted, see the medication dosage calculator and the dose calculator.
Safety: why unit confusion hurts people
In July 2024 the FDA alerted prescribers, compounders and patients to dosing errors with compounded injectable semaglutide. Patients had taken five to twenty times their intended dose, some needing hospital care, mostly by confusing milligrams, millilitres and units when drawing from a multi-dose vial. In one reported case a prescriber meant 0.25 mg, which is 5 units from a 5.00 mg/mL vial, but wrote 25 units, and the patient took five times the dose. Because semaglutide lasts about a week in the body, an overdose does not clear by morning.
Three habits prevent most of these errors. First, always confirm the dose in milligrams, not in units: units are meaningless without the concentration behind them. Second, check the draw backwards, as this article does: units divided by 100 gives millilitres, times concentration gives milligrams, and that number must equal the prescription. Third, treat every new vial as a new calculation, even if it looks identical to the last one, because compounders vary in concentration and a salt form on the label may not be the approved active ingredient at all. Compounded semaglutide has not been reviewed by the FDA for safety, effectiveness or quality, and the STEP trial results for the approved product do not automatically transfer to a compounded preparation. If the pharmacy label, the prescription and this calculator disagree, stop and ask the pharmacist: that disagreement is exactly the situation the FDA warning describes.
What competing calculators miss
Most reconstitution calculators online do the same three divisions and stop there, and the gaps are all around the edges where errors actually happen. Some accept a dose in micrograms but a vial in milligrams without converting, silently mixing units by a factor of one thousand. Some show units without saying which syringe scale they assume, even though a U-40 syringe would read differently. Several present compounded vial math beside brand-name pen schedules as if they were interchangeable, when pen users never measure units at all. And almost none warn that the units number is vial-specific, so a reader memorises one number and reuses it on a stronger vial.
This page fixes the unit handling by keeping the dose in milligrams throughout, naming the U-100 scale on the result itself, separating pen math from vial math in plain language, and printing the ladder table for two vial strengths so the vial-dependence is visible rather than theoretical. It also refuses to invent a feature the old tool did not have: it does not guess at storage life, beyond-use dates or doses per vial, because those depend on the specific pharmacy label, not on arithmetic. What it will not do is tell you a dose: any calculator that picks your semaglutide dose for you is overstepping, and this one does not.
Frequently asked questions
It converts a vial into a syringe draw. From vial strength and water volume it finds the concentration, from your prescribed dose and that concentration it finds the injection volume, and from the volume it finds the U-100 syringe units. It does not choose your dose or your product.
Milligrams measure how much drug there is, millilitres measure how much liquid there is, and units measure a position on a U-100 insulin syringe, where 100 units equals 1 mL. A dose is prescribed in milligrams; units only make sense once you know the concentration of the vial in hand.
Because the concentration changed. A 0.25 mg dose is 10.0 units from a 5 mg vial mixed with 2 mL, but 5.0 units from a 10 mg vial mixed with 2 mL. Units are vial-specific, so every new vial needs a fresh calculation.
No. Brand-name pens are prefilled and the dose is set by the device, so there is nothing to measure. This calculator is only for vials, where the patient draws the dose with a syringe.
For Wegovy injection, the FDA-approved label starts at 0.25 mg once weekly for four weeks, then rises every four weeks through 0.5 mg, 1.0 mg and 1.7 mg to a 2.4 mg maintenance dose. Your prescriber may hold or delay increases based on tolerability, and that decision overrides the label ladder.
No. Compounded semaglutide is not FDA approved and has not been reviewed for safety, effectiveness or quality. Some compounded products use salt forms that differ from the approved active ingredient. The clinical trial results for the approved product do not automatically apply to a compounded preparation.
Stop and ask your pharmacist before injecting anything. A disagreement between the prescription, the pharmacy label and this calculator is exactly the kind of unit confusion the FDA warned about in July 2024, and it is safer to resolve it than to guess which number is right.
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References
- U.S. National Library of Medicine, DailyMed, Wegovy (semaglutide) full prescribing information, including the 0.25 mg to 2.4 mg weekly titration schedule. Accessed 26 October 2026.
- U.S. Food and Drug Administration, Compounding Risk Alerts, including the 26 July 2024 alert on dosing errors with compounded injectable semaglutide. Accessed 26 October 2026.
- GLP-1 Receptor Agonist Toxicity: Semaglutide, Tirzepatide, and Related Agents, Medical Toxicology guidance, including the U-100 conversion and compounded vial error cases. Accessed 26 October 2026.
- National Center for Biotechnology Information, Bookshelf, Semaglutide, LiverTox chapter on mechanism, weekly dosing and history. Accessed 26 October 2026.
- Semaglutide from Bench to Bedside, review of semaglutide design, albumin binding and roughly one-week half-life. Accessed 26 October 2026.