A tirzepatide dosing calculator turns your prescribed dose in mg into units on a U-100 insulin syringe. The formula is simple: mg divided by the vial's mg/mL gives mL, and mL times 100 gives units. At 10 mg/mL, 2.5 mg is 25 units and 5 mg is 50 units. The one number you cannot guess is the concentration, which is printed on your vial label. If you are new to the drug, read our tirzepatide guide first, then use the FormBlends calculator for your own vial.
Key takeaways
- Units = (dose in mg divided by concentration in mg/mL) times 100. This works on any U-100 insulin syringe, where 1 mL equals 100 units.
- At 10 mg/mL, the six labeled doses are 25, 50, 75, 100, 125 and 150 units. At 5 mg/mL the units double. At 20 mg/mL they halve.
- Brand Zepbound and Mounjaro pens and single-dose vials need no calculation. The math only applies to compounded multi-dose vials.
- A calculator converts a dose; it does not choose one. The labeled schedule starts at 2.5 mg for 4 weeks and rises in 2.5 mg steps to a maximum of 15 mg.
- Every new vial can have a new concentration. Recheck the label and redo the math each time.
The three numbers every tirzepatide dose calculator uses
There is no magic in a dosing calculator. It is three inputs and one division.
| Input | Where it comes from | Example |
|---|---|---|
| Dose in mg | Your prescription, following the labeled 2.5 to 15 mg schedule | 5 mg |
| Concentration in mg/mL | The pharmacy label on your vial | 10 mg/mL |
| Syringe scale | U-100 insulin syringe: 1 mL = 100 units | 100 units per mL |
Source: Zepbound and Mounjaro prescribing information for the dose schedule; U-100 syringe standard.
Worked example: 5 mg divided by 10 mg/mL equals 0.5 mL. Multiply 0.5 by 100 and you get 50 units. Draw to the 50 line. That is the whole calculation.
Tirzepatide dosage calculator results at common concentrations
Compounded tirzepatide is most often dispensed at 10 mg/mL, but 5, 15 and 20 mg/mL vials exist. The same 5 mg dose is a different number of units in each. This is why people who switch pharmacies or get a new vial size make errors.
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Try the BMI Calculator →| Dose | 5 mg/mL | 10 mg/mL | 15 mg/mL | 20 mg/mL |
|---|---|---|---|---|
| 2.5 mg | 50 units | 25 units | about 17 units | 12.5 units |
| 5 mg | 100 units | 50 units | about 33 units | 25 units |
| 7.5 mg | 150 units | 75 units | 50 units | 37.5 units |
| 10 mg | 200 units | 100 units | about 67 units | 50 units |
| 12.5 mg | 250 units | 125 units | about 83 units | 62.5 units |
| 15 mg | 300 units | 150 units | 100 units | 75 units |
Source: arithmetic from the labeled strengths (Zepbound and Mounjaro prescribing information) and the U-100 standard. Your vial label controls.
Any result above 100 units will not fit in a 1 mL syringe and needs two draws. Results with a fraction of a unit, like 12.5 or 37.5, need a syringe marked in half units or a rounding rule from your pharmacy. The full mL and units chart lives on the tirzepatide dosage chart.
Why reconstitution changes the answer
Some compounded tirzepatide ships as a powder that you mix with bacteriostatic water. The amount of water you add sets the concentration. Add 1 mL of water to a 10 mg vial and you have 10 mg/mL. Add 2 mL and you have 5 mg/mL. Add 0.5 mL and you have 20 mg/mL. Same vial, three different unit counts for the same dose. Your pharmacy's instructions state the water volume to use, and that volume controls the math. Our page on how to mix tirzepatide powder covers the steps.
What a dosing calculator cannot do
A calculator will happily convert 4 mg or 20 mg into units. Neither is a labeled dose. The Zepbound and Mounjaro labels start at 2.5 mg once weekly for four weeks, increase in 2.5 mg steps after at least four weeks on each dose, and cap at 15 mg. The maintenance doses on the Zepbound label are 5, 10 and 15 mg. A calculator does not know where you are in that schedule, how you tolerated the last step, or what your prescriber wrote. Use it to convert the dose you were prescribed, not to pick a dose.
The GLP-1 plan tool is different. It lays out the labeled week-by-week schedule so you can see when a step-up is allowed. The two tools work together: the plan tells you the mg for the week, the calculator tells you the units for that mg on your vial.
Common calculator mistakes
| Mistake | What goes wrong | Fix |
|---|---|---|
| Confusing mg with mL | 5 mg is not 5 mL. At 10 mg/mL, 5 mL would be 50 mg. | Always divide by concentration first. |
| Using last vial's concentration | A 20 mg/mL vial drawn with 10 mg/mL math delivers double the dose. | Read every new label. Redo the math. |
| Wrong syringe scale | U-40 or tuberculin syringes are marked differently. | Use only a U-100 insulin syringe unless the pharmacy says otherwise. |
| Skipping the water volume | Powder vials have no concentration until mixed. | Follow the pharmacy's water volume exactly. |
| Rounding fractions of a unit | 12.5 units cannot be drawn on a whole-unit syringe. | Ask the pharmacy for a rounding rule or a half-unit syringe. |
Source: arithmetic from the U-100 standard; no clinical claims.
How to check your answer before injecting
Do the math twice. Once on paper, once on the calculator. If they disagree, stop. Then compare the result to the labeled range: at 10 mg/mL, any weekly draw under 25 units or over 150 units is outside the six labeled doses and deserves a call to the pharmacy. Finally, read the number aloud to yourself while you look at the syringe. Our syringe injection guide picks up from there.
What this means for compounded tirzepatide
Compounded tirzepatide is not FDA approved and is not interchangeable with Zepbound or Mounjaro. The unit math above exists only because compounded product comes in multi-dose vials. At FormBlends, compounded tirzepatide is $149 for the first month, then $239 a month, or $214 a month with Subscribe & Save. Injection supplies and shipping are included; lab work is not. The price does not change with dose. Prescriptions are written by independent, state-licensed clinicians of Recess MD LLC, and vials are compounded by state-licensed 503A compounding pharmacies. See pricing or start the assessment at start.
Frequently Asked Questions
Is there a tirzepatide dosage calculator I can use?
Yes. The FormBlends calculator at /calculator takes your dose, your vial concentration or water volume, and returns the draw in mL and units. It converts the dose your prescriber wrote; it does not choose a dose for you.
How does a tirzepatide dosing calculator work?
It divides your dose in mg by the vial concentration in mg/mL to get mL, then multiplies by 100 for units on a U-100 syringe. For 7.5 mg at 10 mg/mL, that is 0.75 mL or 75 units. The concentration comes from your vial label.
What if my vial is not 10 mg/mL?
Use the same formula with your actual concentration. At 5 mg/mL, every dose is double the units of the 10 mg/mL column. At 20 mg/mL, every dose is half. Never carry unit counts from one vial to another without checking.
Can a tirzepatide dose calculator tell me when to increase my dose?
No. The label allows an increase of 2.5 mg only after at least 4 weeks on the current dose, up to 15 mg, and your prescriber decides when. A calculator only converts the number you were given.
How many units is 2.5 mg of tirzepatide?
At 10 mg/mL, 2.5 mg is 0.25 mL, or 25 units. At 5 mg/mL it is 50 units, and at 20 mg/mL it is 12.5 units. Confirm the concentration with your pharmacy before your first draw.
Sources
- Zepbound (tirzepatide) prescribing information, Eli Lilly via DailyMed, revised 08/2026.
- Mounjaro (tirzepatide) prescribing information, Eli Lilly via DailyMed, revised 08/2026.
- Zepbound US prescribing information (PDF), Eli Lilly, 2026.
- FormBlends pricing, Form Blends LLC, verified September 5, 2026.
Next steps
Run your own numbers in the calculator, map your weeks in the GLP-1 plan tool, and keep the tirzepatide dosage chart handy. Browse the tirzepatide hub for every other tirzepatide question. If your math and your prescription disagree, talk to a clinician or your pharmacist before you inject. Compounded tirzepatide is not FDA approved and is not interchangeable with Zepbound or Mounjaro; a licensed provider decides whether it is appropriate for you.
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