Units vs. Milligrams on a GLP-1 Syringe: Why "40 Units" Is Not a Dose

If you searched for how many milligrams 40 units of tirzepatide is, you have almost certainly already found a page that answered with a single number. That number cannot be right, and it is worth understanding exactly why before you draw up a dose based on one.

A unit is a volume. It is not an amount of drug.

The markings on an insulin syringe measure liquid, not medicine. On the U-100 standard that ordinary insulin syringes use, 100 units is one millilitre — so one unit is 0.01 mL, and 40 units is 0.4 mL. That is all the number on the barrel tells you. It says how much liquid you have pulled up. It says nothing whatsoever about how much tirzepatide or semaglutide is dissolved in that liquid.

To turn a volume into a dose you need one more number, and it is not printed on the syringe:

milligrams = units ÷ 100 × concentration in mg/mL

So 40 units of a 5 mg/mL vial is 2 mg. Forty units of a 10 mg/mL vial is 4 mg. Same syringe, same marking, same volume — double the drug. Nothing about the syringe warns you which one you are holding.

The branded products are not sold at one concentration, which is why no table can work

This is the part that makes a universal conversion chart impossible rather than merely imprecise. Tirzepatide's label lists its strengths as 2.5 mg per 0.5 mL, 5 mg per 0.5 mL, 10 mg per 0.5 mL, 12.5 mg per 0.5 mL and 15 mg per 0.5 mL¹. Look at what is being held constant: the volume is 0.5 mL every time, and the milligrams change. That means the concentration changes at every single strength — from 5 mg/mL at the lowest to 30 mg/mL at the highest. The same label separately states concentrations of 4.17, 12.5, 16.7, 20.8 and 25 mg/mL for its other presentations¹.

Semaglutide is built the same way. Its label carries 0.25 mg per 0.5 mL, 0.5 mg per 0.5 mL, 1 mg per 0.5 mL and 2 mg per 3 mL — which is to say 0.68, 1.34 and 2 mg/mL depending on which pen you have².

So there is no such thing as "the" mg-per-unit for either drug. There are at least five for one and three for the other, and that is before anyone compounds anything.

Why none of this comes up with a pen

If you are using an actual Zepbound, Mounjaro, Ozempic or Wegovy pen, this whole problem is invisible to you, by design. You do not measure anything. The pen is built for one strength and dials a dose, and semaglutide's label says so plainly of one presentation: the "pen delivers doses in 1 mg increments only"². You never see a unit, never convert anything, and cannot draw up 40 units of the wrong thing.

The units question only exists because of the other way these drugs are now sold: a vial of compounded drug plus a syringe, with the measuring left to you.

With a compounded vial, the concentration is a decision someone else made

A compounding pharmacy sets its own concentration. It is not obliged to match any brand strength, and different pharmacies do not have to match each other. FDA maintains a standing page on semaglutide products marketed for diabetes and weight loss, including compounded ones, precisely because these are not the approved products and do not carry their labeling³.

The practical consequence: your vial's concentration is a fact about your vial. It should be on the label the pharmacy put on it, usually as something like "5 mg/mL" or as a total strength plus a total volume ("10 mg / 2 mL", which is 5 mg/mL). If you cannot find it, you cannot do this arithmetic, and the honest move is to call the pharmacy and ask rather than borrow a number from a chart written about someone else's product.

The arithmetic, in both directions

Once you have the concentration, both directions are one line each.

Units to milligrams: divide the units by 100 to get millilitres, then multiply by the mg/mL. Fifty units of a 10 mg/mL vial: 50 ÷ 100 = 0.5 mL, × 10 = 5 mg.

Milligrams to units: divide your prescribed milligrams by the mg/mL to get millilitres, then multiply by 100. A 7.5 mg dose from a 10 mg/mL vial: 7.5 ÷ 10 = 0.75 mL, × 100 = 75 units.

If your syringe is not a U-100 insulin syringe, the 100-units-per-millilitre step does not hold and the arithmetic changes. Check the barrel.

Where to check your own numbers

Our reconstitution calculator hub does this conversion on the numbers from your own vial rather than from a table — it converts a dose your prescriber gave you into the volume and units that correspond to it, which is arithmetic, not a recommendation about what your dose should be. For what a compounded vial is and is not, our compounded tirzepatide explainer and our compounded semaglutide explainer cover the difference from the branded pen, and our peptide reconstitution and storage article covers what the labels say about mixing and keeping these products once they are open. Once you know your dose and your volume, where you put the injection is governed by the label for the product you actually have.

What to take from this

The number on your syringe is millilitres wearing a different name. It becomes a dose only when you multiply it by the concentration of the specific vial in your hand, and for these two drugs that concentration is not a constant — the branded labels alone carry five of them for tirzepatide and three for semaglutide. Any page that answers "how many mg is 40 units" with one number has quietly assumed a concentration on your behalf and not told you which one. Find the mg/mL on your own vial, then do the two-step arithmetic above.