Drug Half-Life Calculator
Enter a medicine's half-life and the time since a dose to see roughly how much is left. You'll find the half-life in the drug's prescribing information or patient leaflet.
- Eliminated
- 75%
- ~94% cleared (4 half-lives)
- 24 hours
- ~97% cleared (5 half-lives)
- 30 hours
Half-life by half-life
| Half-lives | Time after dose | Remaining | Amount left |
|---|---|---|---|
| 1 | 6 hours | 50% | 100 mg |
| 2 | 12 hours | 25% | 50 mg |
| 3 | 18 hours | 12.5% | 25 mg |
| 4 | 24 hours | 6.25% | 12.5 mg |
| 5 | 30 hours | 3.13% | 6.25 mg |
| 6 | 36 hours | 1.56% | 3.13 mg |
| 7 | 42 hours | 0.78% | 1.56 mg |
Medical disclaimer. This calculator is for general information and education only. It is not medical advice, not a diagnosis, and not a substitute for professional care. This is a simplified single-dose model. Do not use it to decide when or how much of a medicine to take, or when it is safe to drive, drink alcohol or take another medicine. Real elimination depends on your liver and kidney function, age, other medicines and more. It cannot predict drug test results.Ask your pharmacist or prescriber about your own medicines.Last reviewed .
How half-life works
Most medicines are cleared by first-order elimination: the body removes a fixed fraction of what is present per hour, not a fixed amount. That is what makes half-life constant. Whether the dose was large or small, the amount left halves over each half-life, so the remaining fraction after time t is ½ raised to the power of t ÷ half-life.
The standard pharmacology rule of thumb follows directly: after 4 half-lives about 94% of a single dose is gone, and after 5 about 97%. With regular dosing, the same 4 to 5 half-lives is roughly how long it takes for levels to build up to a steady state, and how long they take to fall away after the last dose.
This simple model leaves things out. It assumes the whole dose is absorbed at once, ignores active metabolites, and uses a single published half-life that may not match yours. Some substances, alcohol most famously, are cleared at a roughly constant rate instead and do not have a meaningful half-life. Treat the results as an illustration of how elimination works, not a measurement of what is in your body.
Frequently asked questions
What is a drug's half-life?
The time it takes for the amount of a drug in the body to fall by half. With a half-life of 6 hours, half of a dose is left after 6 hours, a quarter after 12, and an eighth after 18. It is listed in the clinical pharmacology section of a medicine's prescribing information, usually as a typical value or a range.
How long until a drug is out of my system?
A common rule of thumb in pharmacology is 4 to 5 half-lives, by which point about 94% to 97% of a single dose has been eliminated. For a drug with a 6-hour half-life that is 24 to 30 hours. It is an approximation: some drugs leave active by-products (metabolites) with half-lives of their own, and your own half-life can be longer or shorter than the published figure.
Why does my half-life differ from the published one?
Published half-lives are averages from study groups. How fast you clear a drug depends on how well your liver and kidneys are working, your age, other medicines that speed up or slow down its breakdown, your genes, and sometimes your weight and hydration. For some medicines these effects are large, which is why doses are adjusted for kidney or liver disease.
Does this tell me if I will pass a drug test?
No. Drug tests usually look for metabolites rather than the drug itself, often in urine or hair, and their detection windows depend on the test, its cut-off level, and how much and how often the drug was taken. A half-life calculation cannot answer that question.
Does this work for repeated doses or alcohol?
It models one dose. With regular doses, levels build up and level off at a steady state after about 4 to 5 half-lives. Alcohol does not follow this model at all: the body clears it at a roughly constant amount per hour (zero-order elimination) rather than halving it, so a half-life does not apply.