Drug Half-Life Clearance Calculator — How Long Until It Clears?
Quick answer: Enter the dose, half-life, and hours since dose. See remaining amount, percentage left, and total clearance time.
Calculate how much of a drug or substance remains in your body after a given time, and when it will be effectively cleared. Uses the standard half-life decay model. Works for caffeine, alcohol, common medications, and any substance with a known half-life.
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Last reviewed: April 2026Report an error
Remaining in System
100.00 mg (50.0%)
Of the original 200 mg, 100.00 mg (50.0%) remains after 5 hours. Will be effectively cleared in ~33.2 hours (1.4 days).
Decay Curve Over Time
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How to Use This Drug Clearance Calculator
- 1Enter the dose taken (mg).
- 2Enter the substance's half-life in hours (caffeine ~5, alcohol ~1, ibuprofen ~2).
- 3Enter how many hours since you took it.
- 4Optionally adjust the "cleared threshold" (default 1% of dose).
- 5Read the remaining amount and total clearance time.
Frequently Asked Questions
- The time it takes for half of a drug's amount to be eliminated from your body. After 1 half-life, 50% remains. After 2, 25%. After 5 half-lives, only ~3% remains — usually considered "cleared."
- Caffeine's half-life is ~5 hours. So 200mg at noon = 100mg at 5pm = 50mg at 10pm = 25mg at 3am. Most people clear it in 24-30 hours but 6+ half-lives are needed for full elimination.
- Alcohol is metabolized at a near-constant rate (~0.015% BAC per hour) rather than via half-life. A standard drink takes ~1 hour to fully metabolize. Use a BAC calculator instead for alcohol-specific math.
- Functionally cleared usually means below 1-5% of the original dose. We use 1% as the default. After 5 half-lives, 97% has eliminated.
- No. The half-life model assumes "average" elimination. People with impaired liver/kidney function clear drugs more slowly. Always defer to a doctor or pharmacist for clinical decisions.
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