Half-life calculator: estimate how much is left in your system
Short answer: After each half-life, about half of what was there is gone. With repeated doses, each new dose lands on top of what is left, so levels build up until they level off after about 4 to 5 half-lives. Pick a compound or enter a half-life, set the schedule, and the chart shows an illustrative estimate of the build-up and how long it takes to clear.
How the estimate works
For every dose, the amount remaining after a time t is the dose × 0.5t ÷ half-life. The chart adds up what is left from every dose at each moment. That gives the familiar sawtooth: a jump at each dose, a slow fall, and peaks that rise until the amount lost between doses matches the amount added.
Three numbers on the results are worth knowing:
- Left before each dose: the share of the level that remains when the next dose is due. A long half-life compared with the gap between doses means a flatter curve.
- Build-up: how much higher the steady level is than a single dose, and roughly when you reach it.
- Mostly cleared: about 5 half-lives after the last dose, around 3% remains.
What this estimate leaves out
Real medicines take time to be absorbed, so true peaks arrive later and lower than a simple model shows. People also clear drugs at different speeds, and published half-lives for many research peptides come from animal or very small studies. Treat the chart as a way to understand the shape, not as a number to act on. Only a blood test measures an actual level, and only your prescriber can interpret it.
See your real curve, from your real doses
This page assumes a perfectly regular schedule. Real life has late doses, skipped weeks and dose changes. PepLevel builds the same kind of illustrative estimate from the doses you actually logged, with a marker for right now, for any compound with a half-life. Most people use it to make sense of hunger coming back, side effects on certain days, or the build-up after a dose change. The basic curve is free, and Pro can overlay your weight and side effects on it. It is always labelled as an estimate, never a blood test.
Common questions
How many half-lives until a drug is out of your system?
About 5 half-lives, when roughly 3% remains. That is why a medicine with a 5-day half-life is usually described as taking around 25 days to clear.
Why does the level keep rising with weekly doses?
Each dose is added before the previous one has fully cleared. The level rises until the amount cleared between doses equals the amount added, which takes about 4 to 5 half-lives.
Is this the same as a blood level?
No. It is an illustrative estimate from a published half-life and a regular schedule. It ignores absorption and individual differences, and is not medical advice.
PepLevel does this from your own doses
Saved vials, planned doses in mL, and a level curve built from what you actually logged. Coming soon to the App Store.
Get PepLevelMedical disclaimer: this tool does arithmetic and illustrative estimates only. It is not medical advice, does not recommend any dose and is not a substitute for your prescriber, pharmacist or the product information. PepLevel is a logging and reminder tool only.