Semaglutide vs Tirzepatide vs Retatrutide: Dosing Math

Semaglutide, tirzepatide, and retatrutide get compared constantly because they sit in the same research family — each targets metabolic hormone receptors, each ships as lyophilized powder, and each has to be reconstituted before it can be measured into a syringe. The compounds themselves differ in mechanism and research maturity, but the reconstitution and dosing arithmetic is the same fixed formula across all three. This reference lines up what's different and what isn't. It is written for research and educational purposes only and is not medical advice.

What separates the three compounds

The distinction between semaglutide, tirzepatide, and retatrutide comes down to receptor targets. Semaglutide is a single-receptor GLP-1 agonist. Tirzepatide adds a second target, acting on both GLP-1 and GIP receptors. Retatrutide goes a step further as a triple agonist, acting on GLP-1, GIP, and glucagon receptors. More receptor targets does not automatically mean a stronger or safer compound — it means a different mechanism, studied on a different timeline, with a different depth of evidence behind it.

Semaglutide has the longest research and clinical history of the three, followed by tirzepatide. Retatrutide is the newest and least studied in humans. That order matters more for how confidently a reader should treat any research dosage figure than it does for the reconstitution math itself, which stays identical regardless of mechanism.

Regulatory and research status

Semaglutide and tirzepatide both have FDA-approved pharmaceutical formulations, prescribed under specific brand names for specific approved indications. Retatrutide has no approved formulation and is still moving through clinical trials. That gap in regulatory status is one of the clearest signals of how much human data exists behind each compound: approval requires a body of trial evidence that retatrutide, as an investigational compound, has not yet accumulated. None of the three is discussed here as a product to take — they're research compounds, and this article treats the underlying chemistry and arithmetic, not a use recommendation.

Vial sizes and typical concentrations

Research suppliers list these three in different vial sizes, and each has its own commonly cited reference concentration:

Peptide	Typical vial sizes	Common reference concentration
Semaglutide	2 mg, 5 mg, 10 mg, 15 mg	5 mg/mL (1 mL water per 5 mg)
Tirzepatide	5 mg, 10 mg, 15 mg, 30 mg, 60 mg	10 mg/mL (1 mL water per 10 mg)
Retatrutide	10 mg, 15 mg, 20 mg, 30 mg	10 mg/mL (1 mL water per 10 mg)

These reference concentrations exist because they tend to land common research doses on clean, easy-to-read syringe marks — not because the arithmetic requires them. Any vial size can be reconstituted with any water volume; the peptide calculator recalculates the resulting concentration and units for whatever combination you enter.

Worked example: same formula, three different numbers

The formula behind all three is unchanged: concentration equals vial mass divided by water volume, and syringe units equal that volume in mL multiplied by 100 on a U-100 insulin syringe.

Semaglutide. A 5 mg vial with 1 mL of bacteriostatic water is 5 mg/mL. A 1 mg dose is 1 ÷ 5 = 0.2 mL, or 0.2 × 100 = 20 units.

Tirzepatide. A 10 mg vial with 1 mL of water is 10 mg/mL. A 5 mg dose is 5 ÷ 10 = 0.5 mL, or 0.5 × 100 = 50 units.

Retatrutide. A 10 mg vial with 1 mL of water is also 10 mg/mL. A 4 mg dose is 4 ÷ 10 = 0.4 mL, or 0.4 × 100 = 40 units.

Three different peptides, three different dose sizes, and the same two-step calculation each time: divide for concentration, multiply by 100 for units.

Why one calculator covers all three

Here's how we see it: the step where a research dose actually goes wrong isn't the injection, it's the reconstitution math. A U-100 insulin syringe measures a fixed 100 units per mL, so the mg-per-unit for any peptide depends entirely on how much water went into the vial. Mix up that ratio for semaglutide, tirzepatide, or retatrutide and every draw after is off by the same wrong factor. That's true regardless of which of the three you're working with, which is exactly why a single reconstitution calculator built on the mass-divided-by-volume formula handles all of them without modification. For the full mixing walkthrough, see how to reconstitute peptides.

Where the comparison breaks down

The math is interchangeable; the evidence behind each compound is not. Semaglutide's research and clinical record is the deepest of the three, tirzepatide's is shorter but still substantial, and retatrutide's is the thinnest, reflecting its status as the newest and least-approved of the group. A reference concentration that's well established for semaglutide doesn't carry the same weight of precedent when applied to retatrutide — it's a starting point pulled from a smaller, newer body of research, not a settled standard. Readers comparing the three should weigh dosage figures for retatrutide with that gap in mind rather than treating all three as equally established.

Common mistakes

• Assuming a concentration common for one peptide applies to another. 10 mg/mL is a common reference point for tirzepatide and retatrutide, but semaglutide's is typically 5 mg/mL — using the wrong one doubles or halves every dose calculated from it.
• Treating retatrutide's research maturity as equal to semaglutide's or tirzepatide's. It isn't, and the calculator can't correct for that — it only does the arithmetic you give it.
• Confusing mg and mcg across any of the three. 1 mg equals 1,000 mcg, and reading one for the other produces a 1,000x error.
• Skipping the individual guide for each compound. The vial sizes and reference concentrations above are common defaults, not universal — check the specific guide for the peptide you're working with.

Related

For the compound-specific math, see the semaglutide reconstitution calculator, the tirzepatide reconstitution calculator, and the retatrutide dosage calculator guides. Or enter your own vial size, water volume, and target dose directly into the free peptide calculator to see the exact syringe units for any of the three.