Reverse Dose Calculator: Syringe Units to mg for GLP-1 Vials

1. Choose where you drew the dose from: a compounded vial, a Zepbound or Mounjaro vial, or a KwikPen (Mounjaro prescribing information, section 11), and the drug. The calculator checks the result against that drug’s label steps and maximum.
2. For a compounded vial, type the strength the way your label prints it, such as 20 mg/mL or 10 mg / 0.5 mL.
3. Enter the units in the syringe, or set them on the syringe scale.
4. Add the dose you meant to draw to get a yes or no, with where that dose should sit on the barrel. If the mg does not match, the calculator says by how much and points to the most likely label mix-up.

Dividing the units by 100 gives the volume in mL, and multiplying the volume by the concentration gives the mg: mg = units ÷ 100 × concentration in mg/mL.

You drew 25 units from a vial labeled 20 mg/mL. That is 25 ÷ 100 = 0.25 mL, and 0.25 mL × 20 mg/mL = 5 mg. The same 25 units from a 40 mg/mL vial hold 10 mg.

Each cell is units ÷ 100 × the concentration at the top of the column.

Fifty units is 5 mg on a 10 mg/mL vial and 20 mg on a 40 mg/mL vial, four times as much drug in the same volume. Three cells in the last column are above 15 mg, the top of the tirzepatide label (Zepbound prescribing information, section 2). When a result lands that high, check the concentration you entered before anything else.

One unit is 0.01 mL, so it holds the concentration ÷ 100 in mg.

At 40 mg/mL, reading two units high adds 0.8 mg, about a third of a 2.5 mg dose.

FDA has received reports of people injecting 5 to 20 times their intended dose after drawing compounded semaglutide from multiple-dose vials (FDA alert on compounded semaglutide dosing errors). FDA says confusion between mL, mg and units may have contributed, and in several reports patients told to draw 5 units drew 50. Run the check:

• before you inject a syringe you filled from memory;
• when a new vial or a new pharmacy supplies your medicine, to see what your usual unit count now holds;
• when someone gives you a unit count without a concentration, such as an old note or a friend on the same drug.

When the concentration has changed, check the first draw from the new vial against what changes when your vial strength changes. For a vial made from powder, the concentration depends on the water added; the reconstitution arithmetic shows how.

Your prescription is written in mg, so compare the mg, not the unit count. Branded tirzepatide comes in 2.5, 5, 7.5, 10, 12.5 and 15 mg doses (Zepbound prescribing information, section 3). A compounded prescription can name another amount. FDA has adverse event reports that may be related to compounded semaglutide and tirzepatide prescribed beyond the approved label, such as larger single doses or faster increases (FDA’s concerns with unapproved GLP‑1 drugs).

If the mg does not match your prescription, do not inject it and do not change the draw to force a match. Re-read the concentration on the label first.

Choose semaglutide and the calculator compares the result with the semaglutide maximums: 2 mg once weekly for Ozempic (Ozempic prescribing information, section 2) and 7.2 mg once weekly for Wegovy (Wegovy prescribing information, section 2).

To go the other way, from a prescribed dose to the units to draw, use the tirzepatide dose calculator or the semaglutide dose calculator.

Branded pens deliver a fixed dose and have no unit marks to read. A branded single-dose vial is drawn whole: 0.5 mL, or 50 units, at every strength. A Zepbound multi-dose vial dose is 0.6 mL, or 60 units (Zepbound prescribing information, section 3), so a 50-unit draw from one gives 0.5 ÷ 0.6 = 83% of the dose. The Zepbound vial calculator checks branded vial draws.