Semaglutide dosage calculator: units, mg, and the vial math

Why the answer needs three numbers, not one

Mass on the vial: the milligram figure on a label describes what the vial is supposed to contain, and the insulin syringes this converter covers are graduated in units of volume, not in mass.

Volume on the barrel: a unit is a fraction of a milliliter, fixed by the scale, 0.01mL on U-100 and 0.025mL on U-40, and indifferent to what is dissolved in it.

The diluent decides everything between them: milligrams divided by milliliters yields mg per mL, then mg per mL divided by units per mL yields the mass in one mark. Both divisions are what this page runs, and there is nothing else in it.

The trap the arithmetic exposes: two people holding identical vials can land on unit counts that differ by a factor of four, purely because one added 1mL of water and the other added 4mL. Each reading is correct for the vial it came from. What is wrong is carrying either count across to the other vial, and it is wrong by that same factor of four.

What concentrations do the approved labels print?

Ozempic prints its own strength per mL: the label pairs each multi-dose pen with a concentration figure, so those numbers are label text rather than anything worked out here.

And in all three the printed figure sits above the division: 2mg in 3mL divides to 0.667mg per mL while the label prints 0.68, and the same gap appears on the other two pens. The label figure is the one to work from, because it is the one the manufacturer stands behind.

The other presentations are labelled as an amount in a volume, and two of them also print a strength per mL: section 16 of the Ozempic label prints 0.5, 1 and 2mg per mL for its single-dose prefilled syringes, and the Wegovy label prints 3.2mg per mL for the FlexTouch pen. Everywhere else in the table below, the concentration is division.

The spread is the point: across the two approved injection lines the concentration runs from 0.5mg per mL to 9.6mg per mL, a factor of more than nineteen. A reconstituted vial sits wherever its own arithmetic puts it, and inherits none of these figures.

One worked example, both barrels

The setup: a 5mg vial with 2mL of bacteriostatic water gives 2.5mg per mL, so one U-100 mark holds 0.025mg, which is 25mcg.

Why the U-40 column runs to fractions: the two scales were calibrated for different insulin strengths, and every U-100 count divides by 2.5 to give the U-40 one. A whole U-100 reading therefore stays whole on U-40 only when it is a multiple of five, which none of the first four rows is. That is arithmetic reporting a mismatch, not a recommendation about barrels.

How many units is 2.4mg of semaglutide?

The question has no fixed answer without a concentration attached to it. In the worked example above, a 5mg vial in 2mL sits at 2.5mg per mL, so 2.4mg occupies 0.96mL and reads as 96 units on a U-100 barrel, most of a full 1mL syringe. The same vial reconstituted with 1mL instead sits at 5mg per mL, putting the identical 2.4mg at 48 units. The milligram figure never changed; only the water did.

What does the mg number on a vial promise?

On an approved product it is backed by the label, which pairs the strength with a stated volume and a manufacturer accountable for both. Outside that approval the figure is worth whatever its own supply chain is worth, and the arithmetic downstream inherits exactly that. FDA draws two separate lines here. On compounded drugs it says a compounded drug might be appropriate where a patient's medical need cannot be met by an approved drug or the approved drug is not commercially available, while noting that compounded drugs are not FDA approved and are not reviewed for safety, effectiveness or quality before marketing. Separately, it has warned companies that illegally sold unapproved drugs containing semaglutide falsely labelled for research purposes or not for human consumption, and says those products are of unknown quality and may be harmful. Every unit count on this page assumes the vial contains what it says, because the arithmetic has no way to check.

Why does one milligram figure land on different unit marks?

Because the concentration moved: the same 1mg is 20 units at 5mg per mL and 40 units at 2.5mg per mL, and both are correct for their own vial.

Because the barrel changed: a U-40 mark holds two and a half times the liquid of the same-numbered U-100 mark, so a count borrowed across scales is wrong by that factor in whichever direction it travelled.

Because a chart cannot print a count without fixing all three: a vial size, a water volume and a scale have to be settled before any unit number exists, so a chart that prints one has settled them whether or not it says so. Matching all three against the vial in front of you is what makes two charts agree again.

Does more water mean a weaker dose?

More water means a weaker liquid, not a smaller amount of compound. A 5mg vial holds 5mg whether it is dissolved in 1mL or 5mL, and what changes is how many milliliters carry any given milligram figure, which is to say how far up the barrel the plunger sits. Diluting further spreads any given milligram figure over more marks, and pushes the largest figures past what a 1mL barrel can hold in a single draw. The converter reports both consequences at once: the concentration row, and the volume row.

The rest of the vial math

Around this converter: five more pages in the dose-math library we maintain, each holding a piece this one leaves out.

• Semaglutide dosage guide: the compound reference, with the STEP and SUSTAIN record and the label escalation tables in full.
• Tirzepatide dosage calculator: the same arithmetic against the Mounjaro label's own strength and volume pairs.
• mcg vs mg: the thousandfold slip that turns a plausible unit count into an impossible one.
• How water volume changes your units: the diluent variable, isolated and worked through on its own.
• Bacteriostatic water vs sterile water: what the two diluent labels say, and where they differ.
• The Decadewise briefing: one tool or one label read like this, in your inbox once a week.

How this page is sourced

Label figures come from the labels: the presentation table is read straight off the current Ozempic and Wegovy prescribing information on DailyMed, not from a summary site.

Trial figures come from the abstract: every STEP 1 number above appears in the published abstract, the design figures in its methods and the outcomes in its results, cited below with its DOI and PubMed identifier.

The rest of the numbers are division: the unmarked concentrations, the worked example, and every unit count on the page can be reproduced with a calculator. The regulatory statements are the agency's own wording, attributed where they appear.

Last reviewed: 27 July 2026.

1. 1

   Novo Nordisk. Wegovy (semaglutide) injection and tablets, prescribing information, dosage forms and strengths, and Table 1. Revised June 2026. DailyMed, U.S. National Library of Medicine. Current label.

2. 2

   Novo Nordisk. Ozempic (semaglutide) injection, prescribing information, dosage forms and strengths, and section 2. Revised May 2026. DailyMed, U.S. National Library of Medicine. Current label.

3. 3

   Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. N Engl J Med. 2021;384(11):989-1002. doi:10.1056/NEJMoa2032183. PMID 33567185.

4. 4

   U.S. Food and Drug Administration. FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. fda.gov, drug safety and availability. Agency page.