Half-Life Calculator
Find how much remains after a time from a half-life.
Exponential decay, quantified
A half-life is the time it takes for half of something to disappear — a radioactive isotope, a drug in the bloodstream, a fading signal. After each half-life the amount halves again.
Starting with 100 units and a half-life of 5, after 15 units of time about 12.5 remain — three half-lives. Enter your own values to find the remaining amount at any time.
Precise and private
The calculation runs in your browser, so nothing you enter is uploaded.
Frequently Asked Questions
What is a half-life?
The time for half of a substance to decay or be eliminated. After each half-life, the amount halves again.
What is the formula?
Remaining equals the initial amount times one-half raised to the elapsed time divided by the half-life.
Where is it used?
Radioactive decay, drug elimination, and any exponential decrease.
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