Semaglutide and tirzepatide are the two compounds research peptide buyers ask about most — and the two most often confused with each other. The confusion is understandable: both are long-acting metabolic peptides, both come as lyophilised powder vials in similar sizes, and both are reconstituted the same way. But the amounts discussed for each sit on different scales, and the branded products they come from are dosed completely differently. This page compares the maths, not the medicine, and starts with the warning that matters most: the pens do not belong in this calculator at all.
Pens and branded vials — do not use this calculator
Both peptides are FDA-approved prescription medicines as branded products only, and those products are ready to use. Semaglutide is sold as Ozempic, Wegovy and Rybelsus (approved Dec 2017, June 2021 and Sept 2019); tirzepatide as Mounjaro and Zepbound (approved May 2022 and Nov 2023). The approved products come as pre-filled pen injectors containing solution, or as oral tablets (Rybelsus), or as single-dose vials (tirzepatide 2.5, 5, 7.5, 10, 12.5 and 15 mg, each 0.5 mL, whole vial = one dose). There is nothing to reconstitute: a pen delivers a set amount exactly as the prescriber instructed. This page and its calculator are only for lyophilised powder vials from compounding and research supply. Do not mix the two systems up. The branded strengths — semaglutide pens at 0.25, 0.5, 1, 1.7 and 2.4 mg; tirzepatide at 2.5–15 mg — are product strengths, not powder-vial sizes, and they do not belong in this calculator.
What they are — briefly
Semaglutide is a 31-amino-acid peptide that acts as a single-receptor GLP-1 agonist. Tirzepatide is a 39-amino-acid peptide that acts as a dual GIP + GLP-1 receptor agonist — two incretin pathways in one molecule. That receptor difference is real pharmacology and it is the reason the two are compared in trials (a published head-to-head trial, SURPASS-2, exists; its outcomes are treatment claims and are not stated here). It does not change the reconstitution process: both ship as lyophilised powder, both are mixed with bacteriostatic water, both are stored refrigerated at 2–8 °C once mixed. The maths is the same. Only the numbers entered are different.
The vial sizes overlap — and so do the concentrations
Powder vials of both peptides are commonly sold as 5 mg, 10 mg and 15 mg (semaglutide also 3 mg; tirzepatide also 20, 30 and 60 mg). Take the same vial and the same water for both, and the concentration is identical:
Worked example — both peptides, 5 mg vial, 2 mL of BAC water:
- Concentration (both): (5 × 1,000) ÷ 2 = 2,500 mcg/mL
- Each unit holds 2,500 ÷ 100 = 25 mcg
Same vial, same water, same concentration. The difference appears only when you enter an amount — and the amounts discussed for the two are different.
Worked example — semaglutide, 5 mg vial, 2 mL, example amount 250 mcg:
- Units: 250 ÷ 25 = 10 units (0.1 mL)
- Amounts in the vial: 5,000 ÷ 250 = 20 amounts
Worked example — tirzepatide, 20 mg vial, 2 mL, example amount 2.5 mg:
- First convert the amount: 2.5 mg = 2,500 mcg
- Concentration: (20 × 1,000) ÷ 2 = 10,000 mcg/mL
- Each unit holds 10,000 ÷ 100 = 100 mcg
- Units: 2,500 ÷ 100 = 25 units (0.25 mL)
- Amounts in the vial: 20,000 ÷ 2,500 = 8 amounts
The trap: same vial, different chart
A 5 mg vial of either peptide with 2 mL of water is 2,500 mcg/mL — but a mixing chart written for one does not transfer to the other, because the amounts are different. A 0.25 mg amount of semaglutide is an ordinary figure; a 0.25 mg amount of tirzepatide is ten times smaller than its smallest branded strength. Conversely, a 2.5 mg amount is a normal tirzepatide figure and an enormous semaglutide one. If you use the wrong chart, the syringe maths will still produce a confident-looking answer — it will just be the wrong amount. Always calculate from your own vial label and your own prescribed amount, never from the other peptide’s chart.
A second worked comparison — 10 mg vials, 2 mL of water
- Concentration (both): (10 × 1,000) ÷ 2 = 5,000 mcg/mL
- Each unit holds 5,000 ÷ 100 = 50 mcg
- Semaglutide, example amount 500 mcg (0.5 mg): 500 ÷ 50 = 10 units (0.1 mL)
- Tirzepatide, example amount 5 mg (5,000 mcg): 5,000 ÷ 50 = 100 units (1.0 mL) — a full 1 mL syringe
Same vial, same water, same concentration — and one amount is 10 units while the other fills the syringe. That is why the two peptides’ mixing charts must never be swapped.
Side by side
| Semaglutide | Tirzepatide | |
|---|---|---|
| What it is | 31-amino-acid GLP-1 agonist | 39-amino-acid dual GIP + GLP-1 agonist |
| Branded products | Ozempic, Wegovy, Rybelsus | Mounjaro, Zepbound |
| FDA status | Approved as branded products, prescription-only | Approved as branded products, prescription-only |
| Powder vial sizes | 3 mg, 5 mg, 10 mg (2, 15, 20 mg also cited) | 5 mg, 10 mg, 15 mg, 20 mg, 30 mg, 60 mg |
| Amounts discussed in | Hundreds of mcg (up to ~2.4 mg branded) | Milligrams (2.5–15 mg branded) |
| Example amount | 250 mcg | 2.5 mg (2,500 mcg) |
| Units at 5 mg + 2 mL | 10 units | 100 units (for 2.5 mg) |
| Amounts in a 5 mg vial | 20 (at 250 mcg) | 2 (at 2.5 mg) |
| Reconstitution | BAC water, same process | BAC water, same process |
Storage (commonly cited, both)
Lyophilised powder at −20 °C for long-term storage; once mixed, refrigerate at 2–8 °C, protect from light, do not freeze. How long a mixed vial is said to keep varies by source — most say around 28 days, some say 28–30 days. Treat any window as a planning figure, not a settled fact, and follow your product’s own label.
What the comparison cannot tell you
Which peptide is “better”, which one to use, and whether any amount is safe or appropriate — none of that is a maths question. Both have large published trial programmes behind their branded products, and both are prescription medicines in that form; the powder vials discussed here are not FDA-approved products and are not checked by a regulator for identity or purity. If you are choosing between them, that is a question for a licensed healthcare provider or pharmacist.
Frequently Asked Questions
Is the reconstitution process different for semaglutide and tirzepatide?
No. Both ship as lyophilised powder, both are mixed with bacteriostatic water using the same steps, and the formula is identical. Only the vial sizes and the amounts entered differ.
Can I use a semaglutide mixing chart for tirzepatide?
No. Even when the vial sizes are the same (both 5 mg), the amounts are completely different, and the resulting syringe readings have no overlap. A 0.25 mg amount of semaglutide is an ordinary figure; a 0.25 mg amount of tirzepatide is far below its usual range.
Why is my tirzepatide draw so much bigger?
Because tirzepatide amounts are in milligrams (2.5 mg and up) while semaglutide amounts are in hundreds of micrograms. At the same concentration, a 2.5 mg amount is ten times a 250 mcg amount.
My semaglutide is a pre-filled pen. Does this page apply?
No. Approved pens are pre-mixed and dose in set amounts exactly as your prescriber instructed. This page is only for powder vials you mix with BAC water yourself.
Are the 2.5–15 mg strengths powder vials?
No. Those are the ready-made branded tirzepatide single-dose vials (0.5 mL each, whole vial = one dose). They need no mixing and no calculator.
Which one should I choose?
This site does not answer that. Ask a licensed healthcare provider or pharmacist.
Try the calculator: Peptide Calculator
Related guides
- Semaglutide Calculator
- Tirzepatide Calculator
- Reconstitution Guide
- How Many Doses Are in a Vial?
- mg to mcg Conversion Guide
Educational reference only — not medical advice. This guide describes maths and commonly reported handling steps only; it does not recommend any dose. Always confirm your dose and preparation with a licensed healthcare provider or pharmacist.