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Half-Life Calculator
Find how much remains after a given time from the half-life.
Amount remaining
—
Same units as initial.
| Half-lives elapsed | — |
|---|---|
| Fraction left | — |
| Amount decayed | — |
| — |
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Enter your figures above to see the step-by-step working.
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How the Half-Life Calculator Works
A half-life is the time for half of a quantity to decay. After each half-life, half of what remains is left: one half-life leaves 50%, two leave 25%, three leave 12.5%, and so on. The amount left is the initial amount times one-half raised to the number of half-lives (elapsed time divided by the half-life). Starting with 100 units and a 10-unit half-life, after 30 units of time 12.5 remain.
The same math describes radioactive decay, drug clearance from the body, and any process with exponential decay. Time and half-life just need to be in the same units.
remaining = initial × (½)(time ÷ half-life)
Frequently Asked Questions
How do you calculate half-life decay?
Raise one-half to the number of half-lives elapsed (time divided by the half-life), then multiply by the starting amount.
How much is left after 3 half-lives?
12.5% — each half-life halves what remains: 50%, then 25%, then 12.5%.
Does this work for medication?
Yes — the same exponential-decay math estimates how much of a drug remains as the body clears it, using the drug’s half-life.
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