Tirzepatide Dose Calculator: mg to Units

How the conversion works

Compounded tirzepatide — the same molecule sold as Mounjaro and Zepbound — is drawn from a vial with a U-100 insulin syringe. That syringe is graduated in units of volume: 100 units = 1 mL, so 1 unit = 0.01 mL no matter what is in the vial. The milligrams riding in that volume come entirely from the concentration on the label:

units = (dose in mg ÷ concentration in mg/mL) × 100

Enter your prescribed dose and your vial's mg/mL and the calculator returns three numbers: the unit mark to draw to, rounded to the nearest whole unit; the exact unrounded figure; and the volume in mL. If your vial arrived as powder, switch the concentration control to Reconstituted vial and enter the milligrams of powder and the millilitres of bacteriostatic water — concentration is simply mg ÷ mL.

What branded tirzepatide actually is, in mg/mL

Almost every mg-to-units calculator online reasons from invented concentrations and never tells you what the branded product is, so you have nothing to sanity-check a compounded vial against. Here are the real numbers, straight off the FDA label (Zepbound prescribing information, section 3):

Three things fall out of that table. First, every branded tirzepatide dose is either 50 units or 60 units on a U-100 syringe at any strength — so if a calculator hands you some other number for a branded product, the concentration you typed is wrong. Second, the strongest branded concentration is 30 mg/mL, while compounded vials commonly run 10–40 mg/mL: a compounded vial can legitimately be more concentrated than anything Lilly ships, so the presets on this calculator are not a mistake. Third, the 0.6 mL row is the reason the click question exists at all: a pen holding four doses is the one branded presentation where people start asking what a fraction of a dose would be worth. Lilly's answer is that its Instructions for Use give no method for dialling less than a full dose. That is covered on the Mounjaro click chart for the four-dose KwikPen.

Why a stronger vial needs fewer units

Concentration sits in the denominator, so the relationship runs backwards from intuition: the more concentrated the vial, the fewer units you draw for the same dose. Compounded tirzepatide is usually supplied far more concentrated than the branded product — the presets above span 10 to 40 mg/mL, against 5 mg/mL for a branded 2.5 mg / 0.5 mL vial. The identical 5 mg dose is 50 units at 10 mg/mL and 12.5 units at 40 mg/mL. Four times the difference in what the barrel looks like, no difference at all in the dose. The same arithmetic and the same trap apply to semaglutide mg to units, where compounded concentrations vary just as widely.

Units per dose at four common concentrations

Read the bottom-left corner of that table. A 100-unit syringe holds 1 mL, so at 10 mg/mL a 12.5 mg dose physically will not fit in one draw. That is why concentrations rise as doses climb — and it means a concentration change often lands at exactly the point in titration where you are already changing something else and least likely to notice a third variable.

The label trap that doubles a dose

A vial marked "5 mg / 0.5 mL" is 10 mg/mL, not 5. Halving the volume doubles the concentration. Type 5 into the concentration field from that label and the calculator will tell you to draw 100 units for a 5 mg dose instead of 50 — twice the intended amount. To normalise any label, divide the milligrams by the millilitres.

Reconstitution has the same shape of error: 30 mg of powder in 1.5 mL of water is 20 mg/mL, while the same 30 mg in 1 mL is 30 mg/mL — identical vials, a third fewer units. If your pharmacy has switched you to a different strength, the switching to a different vial concentration calculator does the recalculation for you, and what happens when your pharmacy changes your vial strength walks through why the old unit count stops being safe.

When the answer is not a whole number

At 24 mg/mL a 5 mg dose is 20.8 units, which no U-100 barrel can resolve. The calculator shows 21 units to draw to and keeps the exact figure beside it. Twenty-one units at 24 mg/mL is 5.04 mg — under 1% over, and not a meaningful difference. Rounding is not the risk here. Rounding a number you derived from the wrong concentration is. If you have already drawn and want to check the other direction, you can convert the units you drew back into milligrams.

Reconstituted from powder: the water volume sets the units

This section is arithmetic for a vial your prescriber and pharmacy have already chosen — it is not an instruction to reconstitute anything yourself. The rule is one line: concentration = milligrams of powder ÷ millilitres of water. Everything else follows.

The water volume is the only variable anyone controls here, and it is really a legibility decision. The same 2.5 mg dose is 50 units at 5 mg/mL and 6.3 units at 40 mg/mL — and a six-unit draw leaves no room to misread by a single mark, while a fifty-unit draw does. That is the trade: a dilute vial is easier to measure and empties faster; a concentrated one lasts but punishes a one-unit slip.

What the label says about the dose itself

This tool converts a dose. It does not choose one. Tirzepatide labelling starts at 2.5 mg once weekly, moves in 2.5 mg increments with at least four weeks at each step, and stops at 15 mg once weekly. The recommended maintenance doses are 5, 10 and 15 mg — 7.5 and 12.5 mg are titration steps you pass through, not places to settle. You can map the tirzepatide titration ladder onto your calendar to see when each step could arrive. Tirzepatide's half-life is roughly 5 to 6 days and steady state arrives at four weeks, which is the reason each step is held that long: what you feel in week two is not the level you will be sitting at in week five — the GLP-1 dosage plotter draws that climb. If you are comparing molecules rather than vials, see the full GLP-1 dose chart for Wegovy, Ozempic, Mounjaro and Zepbound.

Where this calculator does not apply

Branded pens and single-dose vials deliver 2.5–15 mg in 0.5 mL — 50 units, fixed, with nothing to select and no syringe to draw (how many mL are in a Zepbound vial covers that presentation in detail). The KwikPen and the multi-dose vial are different: each dose is 0.6 mL, or 60 units, and one pen or vial holds four doses — 2.4 mL in total. That is the standard four-dose format for both brands, and the standard UK Mounjaro presentation, not a Zepbound quirk. Carrying the 0.5 mL habit onto a 0.6 mL presentation delivers about 83% of the intended dose. Unit arithmetic is a compounded-vial problem, and compounded concentrations are standardised neither between pharmacies nor between your own refills. Re-read the mg/mL on every vial, and have your pharmacy confirm your unit count in writing the first time a strength changes.