Half-Life Calculator

After one half-life, half the substance is gone. After two, three-quarters. After ten, only 0.1% remains. Same math for plutonium and ibuprofen.

What's left

Remaining amount

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Percent remaining

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Half-lives elapsed

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Amount decayed

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Time to 1% remaining

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The formula

Remaining = Initial × (1/2)elapsed ÷ half-life. The exponent counts how many half-lives have passed; each multiplies what's left by ½.

Useful half-lives to remember

  • Caffeine: ~5 hours. A 200 mg cup at noon = 100 mg at 5 PM, 50 mg at 10 PM.
  • Alcohol (BAC): roughly 1 hour, though it's actually zero-order (linear, not exponential) at higher concentrations.
  • Carbon-14: 5,730 years — basis for radiocarbon dating up to ~50,000 years.
  • Plutonium-239: 24,100 years — why nuclear waste stays dangerous so long.
  • Iodine-131: 8 days — used in thyroid scans; mostly gone in 2 months.

See also