Milligrams on the script, units on the syringe

Why this tool exists

When the drug is a compounded semaglutide or tirzepatide, the instruction that arrives is almost never in milligrams. It is a mark on a syringe: draw to 40 units. Where the 40 came from generally goes unsaid, which is why so many people end up searching “how many mg is 40 units of semaglutide?” after the fact. That question has no single answer, because it turns entirely on one figure nobody thought to mention: the concentration of your vial, in milligrams per milliliter.

What follows is the arithmetic that should have come with the instruction. Choose the drug, set the concentration printed on your vial, and go whichever way you need: milligrams in for units out, or units in for milligrams out. Underneath, the usual doses and the usual unit counts are already worked out in lookup tables.

The math (so you can verify it yourself)

Nearly every insulin syringe in circulation is a U-100, meaning its markings are set so that 100 units fill 1 mL. That is what US compounding pharmacies ship alongside a GLP-1 vial in all but rare cases [5, 6]. Once that is fixed, the rest is division:

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

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

Worked through: take compounded semaglutide at 2.5 mg/mL, on a starting dose of 0.25 mg a week, and you get

units = (0.25 × 100) ÷ 2.5 = 10 units

— so the plunger stops at the 10-unit mark.

Common compounded concentrations

There are only a handful of concentrations in general use. Across the compounded GLP-1 sellers on our register, two dominate for each molecule:

• Semaglutide: 2.5 mg/mL or 5 mg/mL, and 2.5 is what you will usually be sent. Certain 503A pharmacies move higher-dose patients onto 5 mg/mL so the volume drawn stays reasonable.
• Tirzepatide: 5 mg/mL or 10 mg/mL, and here the stronger one prevails. At 15 mg a week — Zepbound's maintenance dose — anything weaker would put you past 1 mL, which does not fit on one syringe.

Switching concentration in the calculator is one click and everything recomputes as you do it. What it cannot do is read your vial. Check the label first, every time, and let that set the number rather than any figure on this page.

What can go wrong

More dosing mistakes come from one habit than any other: reusing a unit count — from a friend, or from the vial before this one — that was worked out at a different concentration. The same mark on the same syringe carries a different number of milligrams at each concentration, so a change nobody noticed can halve or double the dose while everything still looks exactly as it should. Four things to check:

1. The concentration on the vial moved. A new label reading 5 mg/mL where the last said 2.5 mg/mL means the unit count you have been using now delivers twice the milligrams. Work it out again before you draw.
2. The pharmacy behind it changed. One prescription filled by two compounders can arrive at two different default concentrations. Ask the new one what theirs is.
3. The syringe is not a U-100. What ships with a compounded GLP-1 is, but a U-40 or a U-500 left over from veterinary or insulin supplies will not do the same arithmetic. Read what is printed on the barrel.
4. The answer comes out above 100 units. That is more than 1 mL, and it will not fit in one insulin syringe. Two injections, a stronger vial from the pharmacy, or a 1 mL tuberculin syringe are the ways round it — settle which with your prescriber rather than improvising.

Important safety disclaimer

This page does arithmetic. It does not check that the vial in front of you is the right vial, at the right concentration, in the right syringe, for the right person, at the right time — those remain with your prescriber and with you. GLP Watchdog does not diagnose, does not treat, and does not tell you what to take. If anything about the dose is unclear, do not inject. Call your prescriber or the compounding pharmacy and get it confirmed first.

Related tools and research

• GLP-1 dose plotter: each step of the ladder, plotted as it accumulates toward steady state.
• Compounded semaglutide bioequivalence: the testing a 503A or a 503B is actually obliged to do, and where the quality risk really sits.
• PCAB accreditation investigation: how to judge the pharmacy that fills your script.
• FDA warning letters database: check your seller against enforcement on the record.
• GLP-1 compounded pricing index: what the compounded telehealth market charges right now, end to end.