GLP-1 Peptide Dose Calculator: Semaglutide, Tirzepatide & Retatrutide

How do you calculate a GLP-1 peptide dose in units?

A 5 mg semaglutide vial mixed with 2 mL of bacteriostatic water is 2.5 mg/mL, so an amount of 0.25 mg is 0.1 mL, or 10 units on a U-100 syringe. The rule for any vial: concentration = vial mass ÷ water volume, volume = amount ÷ concentration, units = volume × 100. The calculator below does it for any numbers you type; it is arithmetic, not a dosing recommendation.

Reconstitution calculator

Type your own numbers. They aren't sent anywhere: the math runs in your browser.

Peptide in the vial (mg) Water added (mL)

Amount per draw

mcg mg

Concentration

2.5 mg/mL

Volume to draw

—

U-100 units

—

Type an amount to see the volume and the units.

Arithmetic only. Peptly does not suggest amounts: the amount is yours, from your protocol or the product's documentation. Not medical advice.

What you need

• Lyophilized GLP-1 peptide vial (Semaglutide, Tirzepatide, or Retatrutide)
• Bacteriostatic water for injection (BWFI)
• U-100 insulin syringe (standard for human-medicine GLP-1 dosing)
• Isopropyl alcohol swabs
• A calculator for the math — the one on this page runs in your browser

The reconstitution math, in one screen

All three steps look like this:

• Concentration = vial mass (mg) ÷ BAC water volume (ml). Example: 10 mg ÷ 3 ml = 3.33 mg/ml.
• Draw volume (ml) = target dose (mg) ÷ concentration (mg/ml). Example: 0.5 mg ÷ 3.33 mg/ml = 0.15 ml.
• Units on a U-100 syringe = draw volume × 100. Example: 0.15 ml × 100 = 15 units.

The calculator at the top of this page does this as you type — no scrap paper, no margin for unit-conversion mistakes.

Step-by-step

1. Confirm the vial mass. Most GLP-1 research vials are sold as lyophilized powder in 5 mg, 10 mg, or 15 mg masses for Semaglutide and Tirzepatide. Read the label and confirm total mass in mg.

2. Pick a BAC water volume. A common starting choice: 2 ml BAC water for a 5 mg vial (concentration 2.5 mg/ml), or 3 ml for a 10 mg vial. The right volume is whichever makes your dose land at 10–50 units on a U-100 syringe. The calculator on this page shows the unit count as you change the water volume.

3. Reconstitute slowly. Wipe the rubber stopper with an isopropyl swab. Inject the BAC water down the inside wall of the vial — do not aim at the powder. Swirl gently until dissolved. Never shake.

4. Enter the vial mass and BAC water. In the calculator on this page, type the vial mass (e.g., 10 mg) and the BAC water you actually added (e.g., 3 ml). The concentration works out as 10 ÷ 3 = 3.33 mg/ml.

5. Type your amount. Type the amount in mg or mcg, from your prescription or protocol — the calculator never suggests one. It shows the volume in mL and the U-100 unit count, and draws the line on a syringe. The FDA-label titration steps are listed below for reference.

6. Write it down. Note the vial, the water added and the date on the label, so later draws from the same vial do not need the math again.

Semaglutide dose reference (Wegovy / Ozempic titration)

The FDA-approved Semaglutide titration for weight management (Wegovy) is a 4-week step-up to a maintenance dose:

• Weeks 1–4: 0.25 mg once weekly
• Weeks 5–8: 0.5 mg once weekly
• Weeks 9–12: 1.0 mg once weekly
• Weeks 13–16: 1.7 mg once weekly
• Week 17+: 2.4 mg once weekly (maintenance)

Ozempic (Type 2 diabetes) titrates differently: 0.25 mg → 0.5 mg → 1.0 mg → optional 2.0 mg.

Tirzepatide dose reference (Mounjaro / Zepbound titration)

Tirzepatide is dual-receptor (GIP + GLP-1) and titrates more aggressively. FDA-approved Mounjaro / Zepbound schedule:

• Weeks 1–4: 2.5 mg once weekly
• Weeks 5–8: 5 mg once weekly
• Weeks 9–12: 7.5 mg once weekly (optional)
• Weeks 13–16: 10 mg once weekly
• Weeks 17–20: 12.5 mg once weekly (optional)
• Week 21+: 15 mg once weekly (maintenance, max)

A common research-vial choice for Tirzepatide is 10 mg or 15 mg per vial. At 10 mg + 2 ml BAC (5 mg/ml), a 5 mg dose is 1 ml = 100 units — at the upper end of a 100-unit U-100 syringe. Choose a lower BAC volume (a higher concentration) if you prefer smaller draw volumes per dose.

Retatrutide notes

Retatrutide is a triple-receptor agonist (GIP + GLP-1 + glucagon) still in late-stage clinical research. There is no FDA-approved dosing schedule yet. Published Phase 2 trial doses typically range from 0.5 mg to 12 mg weekly with multi-week titration. Vial masses on the research market commonly come as 5 mg, 10 mg, or 15 mg lyophilized.

The math is the same vial-mass ÷ BAC water as for any peptide. Choose your concentration so the dose you intend to research lands at a readable unit count on a U-100 syringe. Always consult primary literature for current dose ranges.

Common mistakes

• Mixing up mg and mcg. 1 mg = 1,000 mcg. A 0.25 mg dose is 250 mcg, not 2,500.
• Reading U-40 units on a U-100 syringe. Confirm the syringe label says "U-100" before drawing. Cross-reading causes a 2.5× over- or under-dose.
• Using too little BAC water. A high concentration means your dose lands at 2–3 units, which is unreadable. Add more BAC water to spread the dose across 10–50 units.
• Re-deriving the math every dose. Work it out once per vial and write it on the label.
• Shaking the vial. Swirl or roll between palms. Vigorous shaking can damage peptides.
• Mixing Wegovy and Ozempic schedules. Same molecule, different titration: the two labels are not interchangeable.

Why a calculator beats a spreadsheet

• A drawn syringe. The calculator on this page draws the line on a U-100 syringe diagram — you confirm the unit count by eye before pulling.
• No formula slips. Type the vial, the water and the amount; it does the division and the mcg/mg conversion.
• Nothing to install. It runs in your browser, and the numbers you type are not sent anywhere.

Keeping a log of your entries

The Peptly app for iPhone and iPad is a private log, not a calculator: every amount in it is one you typed. Free, with no time limit: log one peptide, with Today and a reminder, injection sites with your next site suggested, your weight with a 30-day trend and the last 30 days of history. Peptly Pro adds stacks, unlimited reminders, your full history with CSV export, Vials and more. No account; your log stays on your phone.

See also

• Peptide calculator
• BAC water calculator
• Peptide reconstitution math reference
• Peptide dosage reference
• How to reconstitute peptides — step-by-step
• Peptide tracker (injection log)

Primary sources

The titration schedules and weight-loss figures referenced here come from the pivotal clinical trials (New England Journal of Medicine) and the FDA-approved prescribing information. The calculator on this page supplies the reconstitution math — not the clinical data, and not a dose recommendation.

• Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP-1) — N Engl J Med, 2021.
• Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1) — N Engl J Med, 2022.
• Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial — N Engl J Med, 2023.
• Dose schedules: FDA-approved prescribing information for Wegovy, Ozempic, Mounjaro, and Zepbound (Drugs@FDA).

Frequently asked questions

What is the standard concentration for reconstituted Semaglutide? +

There is no single standard — concentration is whatever vial mass ÷ BAC water volume you choose. A common research convention: 5 mg vial with 2 ml BAC water gives 2.5 mg/ml, where 10 units on a U-100 syringe = 0.25 mg. The calculator on this page works out the concentration and draw volume for the numbers you type.

How many units is 0.25 mg of Semaglutide? +

It depends entirely on the vial mass and BAC water you used. Example: at 2.5 mg/ml (5 mg vial + 2 ml BAC), 0.25 mg = 0.1 ml = 10 units on a U-100 syringe. At 5 mg/ml (5 mg vial + 1 ml BAC), 0.25 mg = 0.05 ml = 5 units. The calculator on this page does this math for your numbers.

How many units is 5 mg of Tirzepatide? +

Depends on your concentration. Example: at 5 mg/ml (10 mg vial + 2 ml BAC), 5 mg = 1 ml = 100 units on a U-100. At 2.5 mg/ml (10 mg vial + 4 ml BAC), 5 mg = 2 ml = 200 units (you would need two draws). Most research protocols choose a concentration so each dose lands in the 10–50 unit range.

Can I use the same syringe for multiple GLP-1 doses? +

No. Use a new sterile U-100 insulin syringe for each draw. This is a standard injection hygiene rule, not a Peptly recommendation. Reusing syringes risks contamination and dulled needles that increase injection pain.

How long is reconstituted Semaglutide or Tirzepatide stable? +

Stability varies. Most research protocols refrigerate at 2–8 °C and use within 4–6 weeks for Semaglutide and Tirzepatide reconstituted with bacteriostatic water. Consult primary sources or a licensed clinician for stability data specific to your peptide and storage conditions.

What's the difference between U-40 and U-100 syringes for GLP-1 dosing? +

U-100 means 100 units per ml. U-40 means 40 units per ml. The same volume reads as different unit counts on each. The calculator on this page assumes U-100 (the standard human-medicine insulin syringe). Using a U-40 syringe with a U-100 reading will cause a 2.5× overdose. Always confirm your syringe type before drawing.

Why does my draw unit count change when I change BAC water? +

Because concentration changes. More BAC water = lower concentration = larger draw volume for the same dose. Less BAC water = higher concentration = smaller draw. The math is: draw_volume = dose ÷ concentration. The total mass of peptide delivered is the same — only the volume changes.

Does the calculator work for Retatrutide? +

Yes. Retatrutide doesn't have an FDA-approved dosing schedule yet — it's a research peptide — but the reconstitution math is the same (vial mass ÷ BAC water = concentration). You type the amount in mg or mcg, and the calculator on this page returns the draw volume in mL and U-100 units. Always consult primary research literature for dose ranges.