Peptide and GLP-1 Calculators: Dose, Half-Life, Nutrition
PepTrac has twelve free web calculators in three groups: reconstitution and dose math, half-life estimates, and nutrition. Each shows its formula and its sources, converts only the numbers you enter, and does not recommend a dose. The same math runs in the PepTrac iPhone app, which is a free download with an optional Pro plan.
All calculators
- CalculatorReconstitution calculatorVial, water, and dose in. Syringe units out.
- CalculatorPeptide dose calculatorMixed vial in. mg to syringe units, and back.
- CalculatorSemaglutide dose calculatorTurn a semaglutide mg dose into syringe units for a vial.
- CalculatorTirzepatide dose calculatorTurn a tirzepatide mg dose into syringe units, with a fit check.
- CalculatorHalf-life calculatorPercent left, steady state, and an estimated range.
- CalculatorSemaglutide half-lifeAbout 1 week. See what is left after each week.
- CalculatorTirzepatide half-lifeAbout 5 days. How it compares with semaglutide.
- CalculatorRetatrutide half-lifeAbout 6 days in phase 1b. Investigational, not approved.
- CalculatorProtein calculatorBody weight in. A sourced daily protein range out.
- CalculatorGLP-1 protein calculatorTwo ranges side by side: current weight and a reference weight.
- CalculatorTDEE calculatorSee the working: resting energy, activity factor, estimate.
- CalculatorWater intake calculatorReference intakes first, rules of thumb labeled as such.
Which peptide calculator do you need?
Pick the calculator by the question you are asking. If you have a dry vial and need to know how much bacteriostatic water to add, use the reconstitution calculator. If the vial is already mixed and you need to know which mark to read on the syringe, use a dose calculator, and choose the semaglutide or tirzepatide page if you use one of those two. If you want to know how much of a dose is still in your body after a number of days, use a half-life calculator. If your question is about eating, use the protein, calorie, or water calculators. All twelve convert only the numbers you type in. Syringe marks measure volume, and a U-100 insulin syringe has 100 units in 1 mL.[5] None of them chooses a compound or a dose, and none of them replaces your prescriber or the product label. The table lists every calculator with what you enter and what you get back.
| Calculator | You enter | You get | Based on |
|---|---|---|---|
| Reconstitution | Vial mg, water mL, dose | Concentration, mL, syringe units | Division and unit scale |
| Peptide dose | Vial, water, dose in mg or mcg | mL and units, or mg from units | Division and unit scale |
| Semaglutide dose | Vial, water, dose | Units for a vial | FDA label steps shown |
| Tirzepatide dose | Vial, water, dose | Units and barrel fit | FDA label steps shown |
| Peptide half-life | Half-life, time, dose | Percent remaining, range | Exponential decay model |
| Semaglutide half-life | Dose and interval | Estimated level curve | Ozempic label half-life |
| Tirzepatide half-life | Dose and interval | Estimated level curve | Mounjaro label half-life |
| Retatrutide half-life | Dose and interval | Estimated level curve | Published trial data |
| Protein | Weight, age, activity | Daily grams, as a range | Published g per kg ranges |
| GLP-1 protein | Weight, profile | Daily grams, as a range | Published g per kg ranges |
| TDEE | Age, height, weight, activity | Estimated daily calories | Mifflin-St Jeor equation |
| Water intake | Sex, optional weight | Daily total water, liters and oz | National Academies intakes |
How does a peptide reconstitution calculator work?
It uses three formulas. Concentration in milligrams per milliliter equals the vial milligrams divided by the milliliters of water added. The volume to draw equals your dose divided by the concentration. Syringe units equal that volume times the units per milliliter on the syringe. A U-100 insulin syringe has 100 units in 1 mL, and every small notch on a 1 mL barrel is 1 unit.[5] Take a 5 mg vial with 2 mL of water. The concentration is 2.5 mg/mL, so a 0.25 mg dose is 0.1 mL, which is 10 units on a U-100 syringe. A U-40 syringe has 40 units per mL, so the same 0.1 mL reads as 4 units.[6] That is why you set the syringe scale first. The calculators also check that the draw fits the barrel you pick and show the dose that a rounded draw would deliver. The table shows real results.
| Vial and water | Dose | Syringe | Result |
|---|---|---|---|
| 5 mg in 2 mL | 0.25 mg | U-100, 1 mL | 0.1 mL, 10 units |
| 5 mg in 2 mL | 0.25 mg | U-40, 1 mL | 0.1 mL, 4 units |
| 5 mg in 2.5 mL | 0.25 mg | U-100, 1 mL | 12.5 units, between lines |
| 5 mg in 2 mL | 1.5 mg | U-100, 0.5 mL | 60 units, does not fit a 50 unit barrel |
Are the half-life calculators accurate?
They are estimates. Each calculator takes a half-life and applies a standard decay model, so it shows how much of a dose is likely to remain, not what a blood test would measure. The half-lives come from labels or published studies: the Ozempic label gives an elimination half-life of approximately 1 week for semaglutide,[1] and the Mounjaro label gives approximately 5 days for tirzepatide.[2] Levels are shown as relative values, where each dose is scaled so its own peak equals its milligrams. The shaded band is a sensitivity range made by varying the half-life and the time to peak by 20 percent, not a statistical confidence interval. When a compound has no usable half-life, PepTrac draws no curve instead of inventing one. Individual levels vary from person to person and from one injection to the next, so treat the output as a picture of the shape of a curve and not a number to act on.
Can a calculator tell you what dose to take?
No. A calculator converts numbers you supply, and the dose fields on these pages start empty so that no value looks like advice. Your prescriber sets the dose, and the product label or your pharmacist confirms the details. Getting the conversion wrong has real consequences. In a 2024 alert, the FDA reported dosing errors with compounded semaglutide that came from patients measuring wrong doses and from providers miscalculating doses. It described patients told to draw 5 units on a U-100 syringe who drew 50 units, and it reported patients taking five to 20 times the intended dose.[4] Before you draw, confirm three things: the milligrams in the vial, the milliliters of liquid in it, and the scale printed on your syringe. Then check the result a second way, by converting the units back to milligrams. If any number does not match the label, ask your pharmacist.
Do the calculators work for any peptide?
The arithmetic is generic. It needs only mass, volume, and the syringe scale, so it works for any substance sold as a solution you draw with an insulin syringe. What the math cannot tell you is whether a compound is approved, safe, or right for you. The FDA states that compounded drugs are not FDA approved, so the agency does not review them for safety, effectiveness, or quality before they are marketed, and it states that retatrutide and cagrilintide cannot be used in compounding under federal law.[3] A calculator that returns a clean number for such a product is not evidence that the product is legitimate. PepTrac does not sell, prescribe, or supply peptides. It is an educational record-keeping app, and nothing on these pages is medical advice. Use the tools to check arithmetic, and take questions about a compound to your prescriber.
What do the protein, TDEE, and water calculators use?
The nutrition calculators apply published reference values to the weight and activity you enter, and they show a range instead of one number where the research gives a range. The calorie calculator uses the Mifflin-St Jeor equation, which estimates resting energy from weight, height, age, and sex,[8] and multiplies it by an activity factor. The protein calculators multiply body weight in kilograms by grams-per-kilogram ranges taken from published guidance. The water calculator starts from the National Academies figures: women with adequate fluid intake average about 2.7 liters of total water a day, and men about 3.7 liters, with food counting toward the total.[7] The same report says healthy people can meet their hydration needs by letting thirst be their guide.[7] These tools estimate. They do not replace advice from a clinician or dietitian, especially during medical treatment.
Frequently asked questions
Are the calculators free?
Yes. The web calculators are free to use and need no account. The same math is built into the PepTrac iPhone app, which is a free download with an optional Pro plan.
What is a unit on an insulin syringe?
A unit is a volume mark. A U-100 syringe has 100 unit marks per milliliter, so 1 unit is 0.01 mL, and a U-40 syringe has 40 per milliliter. Units say nothing about how much peptide is in the liquid.
What is the difference between mg, mcg, and units?
Milligrams and micrograms measure mass, and 1 mg equals 1,000 mcg. Units on a syringe measure volume. The link between them is the concentration of your vial, so the same units can hold different amounts of peptide.
Do the calculators work for any peptide?
The math is generic and works for any solution drawn with an insulin syringe. It does not know whether a compound is approved or appropriate for you, so confirm that with your prescriber.
Is the PepTrac app required to use the calculators?
No. The web calculators work on their own. The iPhone app adds saving the result to a vial and a schedule, so you can log doses, track vials, and see estimated levels over time.
Is retatrutide approved?
No. The FDA states that retatrutide cannot be used in compounding under federal law. The retatrutide page reports published trial data only and is not a recommendation to use it.
Sources
- DailyMed, National Library of Medicine (Novo Nordisk). Ozempic (semaglutide) injection prescribing information. Accessed October 3, 2026.
- DailyMed, National Library of Medicine (Eli Lilly). Mounjaro (tirzepatide) injection prescribing information. Accessed October 3, 2026.
- U.S. Food and Drug Administration. FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. Accessed October 3, 2026.
- U.S. Food and Drug Administration. FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products. Accessed October 3, 2026.
- MedlinePlus, National Library of Medicine. Giving an Insulin Injection. Accessed October 3, 2026.
- Merck Animal Health. Vetsulin syringe administration for dogs. Accessed October 3, 2026.
- The National Academies of Sciences, Engineering, and Medicine. Report sets dietary intake levels for water, salt, and potassium. Accessed October 3, 2026.
- The American Journal of Clinical Nutrition. A new predictive equation for resting energy expenditure in healthy individuals (Mifflin et al., 1990). Accessed October 3, 2026.
Written and maintained by the team that builds PepTrac. Every calculator on this site runs the same tested math as the app. Read how we research, cite, and update pages in our editorial policy.
