Drug Half-Life Calculator
How much of a medicine is left in your body after a number of hours. Free, private and instant.
How to use the Drug Half-Life
- Enter elimination half-life (hours).
- Enter dose taken (mg).
- Enter time since the dose (hours).
- Press Calculate to get an instant result with a colour scale, chart and plain-English explanation.
About the Drug Half-Life
A medicine's elimination half-life is the time it takes for the amount in your body to fall by half. After 5 half-lives about 97% of a single dose is gone, and with regular doses a medicine reaches a steady level after about 4 to 5 half-lives. This calculator uses the half-life printed in the medicine's leaflet or label to show how much of a dose is left after a given time, with a decay chart.
Also known as: half-life calculator, how long does a drug stay in your system, medication elimination calculator, drug clearance time.
Worked example
- Elimination half-life: 6 hours
- Dose taken: 400 mg
- Time since the dose: 12 hours
Amount left in the body: 100.0 mg (Falling). After 12.0 hours (2.0 half-lives), about 25.0% of the 400 mg dose is still in your body. Most medicines are considered eliminated after about 5 half-lives, which is 30.0 hours for this one.
- Remaining: 100.0 mg (25.0%)
- Half-lives passed: 2.0
- ~97% eliminated after: 30.0 hours
- Steady state with regular dosing: 24.0 hours - 30.0 hours
Formula and cut-offs
Amount remaining = dose x 0.5^(time / half-life). Percent remaining after n half-lives = 100 / 2^n: 50% after 1, 25% after 2, 12.5% after 3, 6.25% after 4 and 3.1% after 5. Steady state with regular dosing takes about 4 to 5 half-lives.
Frequently asked questions
How many half-lives until a drug is out of your system?
About 5 half-lives, when roughly 97% of a single dose has been eliminated. After 7 half-lives less than 1% remains.
Where do I find a medicine's half-life?
It is listed in the patient leaflet or prescribing information, usually under pharmacokinetics or elimination. Your pharmacist can also tell you.
Why might the real half-life be longer?
Older age, liver or kidney disease, other medicines that slow metabolism, and long-term use can all lengthen half-life. Extended-release forms also release the drug over many hours.
Can I use this to know when it is safe to drive or drink alcohol?
No. Effects do not always match blood levels, and individual variation is large. Ask your pharmacist or doctor.
How much of a 400 mg dose is left after 12 hours if the half-life is 6 hours?
Amount left in the body: 100.0 mg (Falling). After 12.0 hours (2.0 half-lives), about 25.0% of the 400 mg dose is still in your body. Most medicines are considered eliminated after about 5 half-lives, which is 30.0 hours for this one. Enter your own numbers above for a personal result.
Sources and references
- Hallare J, Gerriets V. Elimination Half-Life of Drugs. StatPearls (NCBI Bookshelf)
- National Kidney Foundation - eGFR calculator and CKD staging
- NHS - Chronic kidney disease
Written and checked by the MediHub editorial team against the sources above. Read how we build and review our calculators.
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Last updated . This calculator is for education only and is not medical advice. See the medical disclaimer.