Peptide Half-Life Calculator: Percent Left, Steady State, Range
A peptide half-life calculator estimates how much of a dose remains using remaining = dose x 0.5 to the power of (time divided by half-life). After one half-life 50 percent remains, and after five about 3 percent. With repeated dosing, steady state arrives after roughly four to five half-lives. PepTrac shows the result as an estimate with a range, not one number.[4]
Half-life calculator
Estimate, not a lab result. Levels are relative: each dose is scaled so its own peak equals its mg. The shaded band is a sensitivity range, not a confidence interval.
How do you calculate how much of a peptide is left?
Use the formula remaining = dose x 0.5 raised to the power of (time elapsed divided by half-life). The half-life and the time must use the same unit. Take a half-life of 168 hours, which is 7 days, and a 1 mg dose measured 10 days, or 240 hours, later. The exponent is 240 divided by 168, which is 1.43 half-lives. Half raised to that power is 0.372, so about 37.2 percent, or 0.37 mg, is still present. The formula assumes the drug is fully absorbed and then clears at a steady rate, which is a simplification. It also works for any substance with a known half-life, so it does not need to be a peptide. Real bodies differ, and the formula gives an average-person estimate, not a measurement of your own blood. The calculator above does this for you. Type a half-life, choose hours or days, enter an interval, and it shows the share left and a level curve.
How many half-lives until a peptide is out of your system?
No amount of halving reaches exactly zero, so the practical answer is four to five half-lives. After four half-lives, 0.5 to the fourth power leaves 6.25 percent, which means 93.75 percent has cleared. After five, 3.1 percent remains and 96.9 percent has cleared. Pharmacokinetic papers describe about five half-lives as the span over which repeated dosing settles or washes out.[4] What counts as out of your system depends on how little of a drug still matters, and that differs for every drug. A long half-life stretches the whole timeline, so a drug with a half-life of days takes weeks to clear, and one with a half-life of minutes clears within hours. The table converts half-lives into days for three weekly-injection GLP-1 drugs, using half-lives from their labels or trials. Retatrutide is investigational, and its half-life is a single approximate trial value, so treat that column as an illustration only.[3]
| Half-lives | Remaining | Steady state reached | Semaglutide (7 d) | Tirzepatide (5 d) | Retatrutide (6 d) |
|---|---|---|---|---|---|
| 1 | 50% | 50% | 7 d | 5 d | 6 d |
| 2 | 25% | 75% | 14 d | 10 d | 12 d |
| 3 | 12.5% | 87.5% | 21 d | 15 d | 18 d |
| 4 | 6.25% | 93.75% | 28 d | 20 d | 24 d |
| 5 | 3.1% | 96.9% | 35 d | 25 d | 30 d |
| 6 | 1.6% | 98.4% | 42 d | 30 d | 36 d |
| 7 | 0.8% | 99.2% | 49 d | 35 d | 42 d |
What is steady state and how long does it take?
Steady state is the point during regular dosing where the amount you take in each interval equals the amount your body clears, so the level before each dose stops climbing. It takes about four to five half-lives, whatever the dose size.[4] The Ozempic label shows this in practice: semaglutide has a half-life of approximately 1 week, and steady-state exposure is achieved following 4 to 5 weeks of once-weekly administration.[1] The Mounjaro label gives about 5 days and steady state after 4 weeks.[2] A different dose changes how high the plateau sits, but not how long it takes to arrive. The build-up ratio at steady state is 1 divided by (1 minus 0.5 to the power of interval over half-life). With a weekly interval, a 7 day half-life gives 2.00, a 6 day half-life gives 1.80, and a 5 day half-life gives 1.61. Longer half-lives build up more.
Why does PepTrac show a range instead of one number?
Because the inputs are never exact. A published half-life is an average from a study, and real people clear drugs faster or slower. PepTrac therefore runs its model with the half-life 20 percent lower and 20 percent higher, and with the time to peak 20 percent lower and 20 percent higher. That makes nine combinations. It then draws the lowest and highest results as a shaded band around the main line. The band is a sensitivity range, meaning it shows how much the answer moves if the inputs move. It is not a statistical confidence interval, and it does not say that 95 percent of people fall inside it. Levels are also relative. Each dose is scaled so its own peak equals its milligrams, because PepTrac cannot know the bioavailability or volume of distribution of an individual. The estimate is for education and record keeping, and it is not a lab result.
What happens when a peptide has no time-to-peak data?
The calculator still draws a curve, but it says so and widens the band. When you type a custom half-life, no time to peak is supplied. PepTrac then assumes the peak comes at 0.2 times the half-life, which is 33.6 hours for a 168 hour half-life, and it sweeps that assumption by 50 percent instead of 20 percent. The curve is flagged as assumed. Compare the two results for one 1 mg dose with a 7 day half-life. With no peak time, the modeled level at day 7 is 0.60, with a range of 0.47 to 0.71. With a supplied 48 hour peak, it is 0.65, with a narrower range of 0.55 to 0.73. At day 14 the assumed version reads 0.30, with a range of 0.20 to 0.40, and at day 28 it reads 0.07, with a range of 0.03 to 0.13. The wider band is the honest way to show missing information.
| Day | Assumed peak, 33.6 h | Supplied peak, 48 h |
|---|---|---|
| 7 | 0.60 (0.47 to 0.71) | 0.65 (0.55 to 0.73) |
| 14 | 0.30 (0.20 to 0.40) | 0.33 (0.23 to 0.41) |
| 28 | 0.07 (0.03 to 0.13) | 0.08 (0.04 to 0.13) |
Which peptide half-lives come from human data?
Not all of them, and the difference matters. The GLP-1 drugs have human half-lives from prescribing information or trials: about 1 week for semaglutide in the Ozempic label,[1] about 5 days for tirzepatide in the Mounjaro label,[2] and about 6 days for retatrutide in a phase 1b trial.[3] Ipamorelin has a human value of about 2 hours, but from an intravenous infusion study in healthy volunteers.[7] Thymosin beta-4 ranged from 0.5 to 2.08 hours in an intravenous phase I study of a recombinant form.[6] BPC-157 has only animal data, with a half-life under 30 minutes in rats and dogs and no human data in that study.[5] An animal half-life is not a human half-life, and an intravenous value does not describe an injection under the skin. PepTrac labels the source and lowers confidence accordingly.
| Compound | Half-life | Route and species | Source |
|---|---|---|---|
| Semaglutide | about 1 week | Subcutaneous, human, label | Ozempic label[1] |
| Tirzepatide | about 5 days | Subcutaneous, human, label | Mounjaro label[2] |
| Retatrutide | approximately 6 days | Human, phase 1b, type 2 diabetes. Investigational | Lancet 2022[3] |
| Ipamorelin | about 2 hours | Intravenous, human volunteers | Pharm Res 1999[7] |
| Thymosin beta-4 (TB-500 in the catalog) | 0.5 to 2.08 hours | Intravenous, human, recombinant form | J Cell Mol Med 2021[6] |
| BPC-157 | under 30 minutes | Intravenous and intramuscular, rats and dogs only | Front Pharmacol 2022[5] |
Does half-life tell you how long effects last?
No. Half-life describes how fast the amount of a drug in the body declines. It does not describe when an effect starts, how strong it is, or how long it lasts. Two drugs with the same half-life can act very differently, because effects depend on how the drug binds to its targets and on how the body responds. A longer half-life does not mean a stronger drug either. It mostly affects how often a drug needs to be taken and how long it takes to settle at steady state. The Ozempic label, for example, ties its once-weekly schedule to a half-life of approximately 1 week and a presence in the circulation of about 5 weeks after the last dose.[1] That is a statement about concentration. This calculator is educational and never suggests a dose, schedule, or product. Your prescriber and the product label decide those.
Frequently asked questions
Is half-life the same as duration of action?
No. Half-life is how long it takes the amount of a drug in the body to fall by half. Duration of action is how long an effect lasts, which depends on more than concentration. Ask your clinician about effects.
Why is the curve above 50 percent at one half-life after injection?
The model includes absorption. After an injection the level rises to a peak before it falls, so the decline starts late. With a 7 day half-life and a 48 hour peak, the level at day 7 is 0.65 of the peak, not 0.50.
Can I enter my own half-life?
Yes. Type any half-life in hours or days. A custom value has no time to peak, so PepTrac assumes one at 0.2 times the half-life, widens the band, and flags the curve as assumed.
Why is there no curve for my compound?
Without a usable half-life, PepTrac returns no estimate and draws no curve. It would rather show nothing than invent a number. Add a half-life that you can source, and the curve will appear.
Are these blood levels?
No. Levels are relative, with each dose scaled so its own peak equals its milligrams. PepTrac cannot know your bioavailability or volume of distribution, so the curve shows shape and timing, not a lab concentration.
Does a longer half-life mean a stronger drug?
No. A longer half-life means the amount in the body falls more slowly, so the drug is usually dosed less often and takes longer to reach steady state. It says nothing about strength or results.
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.
- The Lancet (via PubMed, National Library of Medicine). LY3437943, a novel triple GIP, GLP-1, and glucagon receptor agonist in people with type 2 diabetes: a phase 1b trial. Accessed October 3, 2026.
- Drug Design, Development and Therapy (via PubMed Central). Contribution of Trough Concentration Data in the Evaluation of Multiple-Dose Pharmacokinetics for Drugs with Delayed Distributional Equilibrium and Long Half-Life. Accessed October 3, 2026.
- Frontiers in Pharmacology (via PubMed Central). Pharmacokinetics, distribution, metabolism, and excretion of body-protective compound 157 in rats and dogs. Accessed October 3, 2026.
- Journal of Cellular and Molecular Medicine (via PubMed Central). A first-in-human phase I study of recombinant human thymosin beta-4 in healthy Chinese volunteers. Accessed October 3, 2026.
- Pharmaceutical Research (via PubMed, National Library of Medicine). Pharmacokinetic-pharmacodynamic modeling of ipamorelin, a growth hormone releasing peptide, in human volunteers. 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.
