Drug Half-Life Calculator
Elimination half-life, rate constant and amount remaining over time.
Enter your values to see the result.
Assumes first-order (exponential) elimination: t½ = ln 2 ÷ k. About 97% of a drug is eliminated, and steady state is about 97% reached, after 5 half-lives. For education and clinical decision support only. Results do not replace professional judgment, local protocols or the product information, so always verify doses and values before acting on them.
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- Pharmacokinetic Calculator — Analyse a full concentration–time profile.
- Maintenance Dose Calculator — Plan regular dosing.
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Clear instructions
Find steps, examples and limitations below.
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How to use the Drug Half-Life Calculator
Elimination half-life, rate constant and amount remaining over time.
- 1 Choose half-life from two levels, from k, k from half-life, or amount remaining.
- 2 Enter the concentrations and times, k, or the half-life.
- 3 Read the result and the table of half-lives.
Example and practical tips
Levels of 24 mg/L at 1 h and 9 mg/L at 7 h give k = 0.163 per hour and a half-life of 4.24 hours.
Frequently asked questions
How long until a drug is gone?
About 5 half-lives remove roughly 97% of the drug. The same time is needed to reach steady state with regular dosing.
Does this work for every drug?
It assumes first-order (exponential) elimination. Drugs such as phenytoin and alcohol at high levels do not follow it.
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