Half Life Calculator · Practical guide
Drug Half-Life Calculator
How fast a medication leaves your system follows the same half-life math as radioactive decay.
At a glance
What Drug Half-Life Means
A drug's half-life is the time for its concentration in the bloodstream to fall by half. If a medication has an 8-hour half-life, a 200 mg dose leaves 100 mg after 8 hours, 50 mg after 16 hours, and 25 mg after 24 hours — three half-lives, 12.5% remaining.
Interactive tool
Half Life Calculator
Enter your own values, then compare the result with the examples below.
Result
Clearance Over Time (8-Hour Half-Life, 200 mg Dose)
| Time After Dose | Half-Lives | Amount Remaining |
|---|---|---|
| 8 hours | 1 | 100 mg |
| 16 hours | 2 | 50 mg |
| 24 hours | 3 | 25 mg |
| 40 hours | 5 | 6.25 mg |
Use the calculator above with any dose and half-life — set the units independently (hours for the half-life, days for elapsed time) and it converts automatically.
When Is a Drug Effectively Gone?
Clinicians generally consider a drug cleared after about 4 to 5 half-lives, when 94-97% has left the body. For a drug with a 6-hour half-life that is roughly a day; for one with a 36-hour half-life, about a week.
Why Half-Life Shapes Dosing Schedules
Short half-lives require frequent dosing to keep levels steady; long half-lives allow once-daily or weekly pills but take longer to reach steady state and longer to wash out. This is informational math only — never adjust any prescription based on a calculator.
Frequently Asked Questions
Is drug half-life the same as duration of effect?
No. Effects can fade faster or last longer than the blood level suggests, depending on how the drug acts and where it accumulates. Half-life measures concentration only.
How long until 90% of a drug is eliminated?
About 3.3 half-lives: after 3 half-lives 12.5% remains, and (0.5)^3.3 ≈ 0.10, so roughly 90% is gone. Five half-lives clears about 97%.
Does everyone clear drugs at the same rate?
No. Age, liver and kidney function, and drug interactions all shift effective half-life, which is why dosing guidance is individualized by a prescriber.
Can I use this to time my next dose?
The math shows how levels decline, but dosing schedules account for much more. Always follow the label or your prescriber's instructions.