Half-Life Calculator
Work out how much of a substance remains after a number of half-lives.
Decay by halving
A half-life is the time it takes for half of a decaying substance to disappear — after one half-life, half remains, after two, a quarter, and so on. This works out how much is left after a given time, or how long until a certain amount remains, the key calculation for radioactive decay and anything that decays exponentially.
Enter the half-life and time to see how much remains.
Exponential decay is counterintuitive
The surprising thing about half-life decay is how slowly the tail vanishes. Each half-life removes half of what is left, not a fixed amount, so the substance never quite reaches zero — after ten half-lives about a thousandth remains, after twenty about a millionth. This is why some radioactive materials stay hazardous for so long, and it is the same maths behind carbon dating, which reads how much of a radioactive isotope is left to estimate age.
Half-Life Calculator FAQ
How much remains after several half-lives?
Half after one, a quarter after two, an eighth after three — each half-life removes half of what is left, so it approaches but never reaches zero.
Where does half-life apply beyond radioactivity?
Carbon dating uses it to estimate age, and medicine uses it to describe how long a drug remains in the body.
Is anything uploaded?
No. The calculation happens in your browser.