Tirzepatide Dosage Calculator – Units on a Syringe
Convert a prescribed tirzepatide (or semaglutide) dose in mg into the volume in mL and units to draw on a U-100 insulin syringe. Enter your dose, vial concentration and syringe size to see the exact syringe marking, doses per vial and how long it lasts.
Convert a Dose in mg to mL and Units on a U-100 Syringe
Tirzepatide Dose Formula
- \(V\) — volume to draw, in mL
- \(D\) — prescribed dose, in mg
- \(C\) — vial concentration, in mg/mL (either stated directly or \(C = \) medication per vial \(\div\) water added)
- \(U\) — units to draw on a U-100 syringe
Tirzepatide Dose – Calculation Example
Example: you were prescribed a once-weekly 2.5 mg tirzepatide dose. Your vial holds 10 mg of medication in 2 mL of liquid after reconstitution, and you are using a 0.5 mL (50-unit) U-100 syringe.
\[ C = \frac{10 \text{ mg}}{2 \text{ mL}} = 5 \text{ mg/mL} \qquad V = \frac{2.5 \text{ mg}}{5 \text{ mg/mL}} = 0.5 \text{ mL} \qquad U = 0.5 \times 100 = 50 \text{ units} \]
\[ \text{doses per vial} = \frac{10 \text{ mg}}{2.5 \text{ mg}} \approx 4 \text{ doses} \qquad 4 \times 7 = 28 \text{ days} \]
Result: draw 0.5 mL, which is exactly 50 units on the syringe — the full scale of the 0.5 mL syringe. One vial provides 4 weekly doses and lasts 28 days.
How to use the calculator
- Select the Medication — this only changes the titration schedule shown; the math is identical for any medication.
- Enter your Prescribed dose in mg, exactly as written on your prescription (the tirzepatide starting dose is 2.5 mg once weekly).
- Choose the Syringe size you will actually use: 0.3 mL (30 units), 0.5 mL (50 units) or 1 mL (100 units).
- Pick how you know the concentration under How do you know the concentration? If the pharmacy or vial label states mg per mL, choose “I know mg per mL” and enter it. If you reconstituted the vial yourself, choose “I know vial mg + water added” and enter the Medication per vial in mg and the Bacteriostatic water added in mL.
- Enter the Total liquid in vial in mL (used to estimate doses per vial).
- Read the Volume to draw, Units on the syringe, and check the syringe drawing, where the red line marks where to stop the plunger.
What the result means
Tirzepatide (brand names Mounjaro and Zepbound) and semaglutide (Ozempic, Wegovy) are once-weekly injections usually given with prefilled pens at fixed strengths. When the medication comes from a compounded vial or a vial that must be reconstituted, there is no pen to set the dose — you have to calculate a volume yourself and draw it with an insulin syringe. The dose in milligrams is fixed by your prescriber, but the volume that corresponds to that dose depends entirely on how much medication is dissolved in each milliliter of your vial.
The volume result tells you how many mL to draw; the units result tells you the same thing in the language of the syringe. Because U-100 syringes define 100 units as 1 mL, 1 unit always equals 0.01 mL. A 0.5 mg dose from a 2.5 mg/mL vial is 0.2 mL, or 20 units — the same 20-unit mark used for 20 units of insulin. The scale is about volume, not about the strength of the medication inside.
The calculator also tells you how many full weekly doses one vial contains and how many days it lasts, so you can plan refills and avoid running out mid-titration. The syringe drawing shows the red fill line at your dose; the practical notes give the nearest readable marking, since 0.3 mL syringes have half-unit marks while larger ones are typically marked in whole units.
Concentration is everything — and the main risk
A small error in concentration turns into a large error in dose. If you assume 5 mg/mL but the vial really contains 10 mg/mL, you inject double the intended dose; if it is weaker, you inject too little. With GLP-1 medicines, a wrong concentration can easily mean a 2–10 fold overdose. Before drawing anything, confirm the concentration with your pharmacist or the vial label — never guess it from the vial size alone.
Likewise, never mix two syringes worth of medication to hit a dose that exceeds syringe capacity without professional guidance, and never switch vials of different concentration mid-titration without recalculating. If the result seems off — for instance, the starting 2.5 mg dose fills the whole syringe — stop and verify the numbers before injecting.
Some practical tips: draw slowly, expel air bubbles, and double-check the plunger against the scale at eye level. Respect storage rules: refrigerate the vial as instructed, use it within the stated time after reconstitution, and never reuse needles or share vials. If anything is unclear, take the vial and the syringe to your pharmacist and ask them to walk you through one dose in person.
This calculator is a calculation aid only, not medical advice. Dose changes, titration steps and missed-dose rules belong to your prescriber. If you suspect an overdose or develop severe nausea, vomiting, or abdominal pain after an injection, seek urgent medical advice.
Managing reconstituted vials and related tools
If you reconstitute vials regularly, the Peptide Reconstitution calculator helps plan how much bacteriostatic water to add for a target concentration. For liquid medicines dosed in mg/kg or mL rather than fixed weekly injections, see the Liquid Dose calculator, and for checking whether it is time for the next dose after a missed one, the Half-Life Time calculator covers the underlying timing arithmetic.
Frequently Asked Questions
How many units is my tirzepatide dose in mg?
There is no fixed conversion, because units measure volume, not the medicine itself. First find your vial's concentration: a 2.5 mg dose from a 5 mg/mL vial is \(2.5 \div 5 = 0.5\) mL, which is 50 units, while the same 2.5 mg from a 10 mg/mL vial is only 25 units. Always divide your mg dose by your vial's mg/mL and multiply by 100 to get the units to draw. Confirm your concentration with your pharmacist before injecting.
Do the units on an insulin syringe mean milligrams or international units (IU)?
Neither. The U-100 scale is purely a volume scale: 100 units = 1 mL, so 1 unit = 0.01 mL, no matter what liquid is inside. Reading "20 units" does not mean the syringe contains 20 mg of tirzepatide or 20 IU — it means 0.2 mL. That is why the same dose in mg produces different unit readings when the vial concentration changes. Confusing units with milligrams is one of the most common and dangerous errors with reconstituted GLP-1 vials.
What if my dose needs more units than my syringe holds?
If the calculated volume exceeds your syringe's capacity (30, 50 or 100 units), do not simply split the dose into two separate injections on your own — combined draws are hard to measure accurately and increase error risk. Instead, ask your pharmacist for a larger 1 mL syringe or a more concentrated vial. If the numbers unexpectedly don't fit, treat that as a warning sign that the concentration may be different from what you assumed, and verify before injecting.
Why does my friend with the same prescription draw a different number of units?
Because their vial was probably mixed with a different amount of bacteriostatic water. A 10 mg vial reconstituted with 2 mL yields 5 mg/mL, but the same vial with 1 mL yields 10 mg/mL — so the same 2.5 mg dose is 50 units for one person and 25 units for the other. The mg dose is what your prescriber sets; the units depend entirely on your own vial's concentration. Never copy someone else's unit measurement without checking their concentration matches yours.
How much bacteriostatic water should I add when reconstituting a vial?
The amount of water does not change the total mg of medication in the vial — it only sets the concentration and therefore the volume per dose. More water means a weaker solution and a larger, more accurate draw; less water means smaller volumes that are harder to measure precisely, especially on syringes without half-unit marks. Whatever volume you choose, record it and use the same concentration for every dose from that vial. Follow the storage and usage-time instructions from your pharmacist after mixing.
How do I draw a dose that falls between two syringe marks?
Small doses, like 0.25 mg semaglutide from a concentrated vial, often land between markings. The 0.3 mL (30-unit) syringe has half-unit marks (0.005 mL), giving the best precision, while 0.5 mL and 1 mL syringes are typically marked in whole units. Position the plunger tip as close to the calculated mark as possible, at eye level, and never round up — with strong vials even one unit can be several tenths of a milligram of medication. If you cannot read the needed mark clearly, ask your pharmacist to demonstrate.
Can I use the leftover medication in my vial for another dose?
Only if the leftover amount covers a full prescribed dose and the vial is still within its safe-use period. Divide the total mg in the vial by your dose to see how many complete doses it contains; a remainder too small for a full dose should not be topped up from a second vial, since concentrations may differ. Track when you reconstituted each vial, refrigerate it as instructed, and discard it after the stated number of days — never pool vials or share medication with another person.