Half-life calculator
Type a half-life and how long ago the amount was, or pick a medicine to fill in the half-life from its label. You get the estimated share left, and the amount left if you type one.
Share left = 0.5 ^ (time since ÷ half-life)
Or type any half-life below.
This estimates what's left of one amount. It doesn't say when or how much to take next. Ask your prescriber.
Worked example
With a half-life of 5 days, 25% is left after 10 days (2 half-lives) and about 3% after 25 days (5 half-lives). If you type an amount of 2 mg, 0.5 mg is left after 10 days.
How the math works
Each half-life halves what's left, so after n half-lives, 0.5 to the power n is left. The page works it out as e^(−0.693 × time ÷ half-life), which is the same number. It's the formula the PepBloom app's estimated-level chart uses for each logged amount.
It's an estimate for one amount, from an average half-life. Real levels differ from person to person and the curve isn't a blood test. StatPearls explains how half-life is measured.
It runs in your browser. Nothing you type leaves this page, and nothing is saved.
Half-lives on each label
| Medicine | Half-life | What the label says (section 12.3) |
|---|---|---|
| Semaglutide (Ozempic, Wegovy) | About 7 days | "an elimination half-life of approximately 1 week" (Wegovy label) |
| Tirzepatide (Mounjaro, Zepbound) | About 5 days | "the half-life of tirzepatide of approximately 5 days" (Zepbound label) |
| Liraglutide (Saxenda, Victoza) | About 13 hours | "a plasma half-life of 13 hours after subcutaneous administration" (Saxenda label) |
Checked against each label on DailyMed on Nov 6, 2026. Compounded and research products have no FDA label, so they aren't listed. Type their half-life yourself only if it comes from a source you trust.
Questions
What does half-life mean?
It's the time it takes for the level in the blood to fall by half (StatPearls). After one half-life 50% is left, after two 25%, and after five about 3%, because 0.5 multiplied five times is 0.031.
How long does tirzepatide stay in your system?
Its Zepbound label gives a half-life of about 5 days, so about 3% is left after 25 days by this arithmetic. More on the tirzepatide page.
Does this tell me when to take my next dose?
No. It only estimates what's left of one amount you type. When and how much to take come from your prescriber and your prescription label.
Sources
- Wegovy (semaglutide) prescribing information DailyMed
- Zepbound (tirzepatide) prescribing information DailyMed
- Saxenda (liraglutide) prescribing information DailyMed
- Hallare J, Gerriets V. Elimination Half-Life of Drugs. StatPearls, 2025 pubmed.ncbi.nlm.nih.gov
Related
semaglutide (Ozempic, Wegovy) · tirzepatide (Mounjaro, Zepbound) · liraglutide (Saxenda) · Weight loss percentage · Vial run-out date
PepBloom draws this curve from your own log
Log each shot and see an estimated level chart built from every amount.