Half-Life Calculator
Calculate how much of a radioactive isotope or drug remains after a given time — or the half-life or time from the amounts — with exponential decay.
How to use
- 1Choose what to find.
- 2Enter the other values in the same time unit.
How it's calculated
N = N₀ × (1/2)^(t ÷ t½). Half-life t½ = t × ln 2 ÷ ln(N₀ ÷ N). Time t = t½ × log₂(N₀ ÷ N).
Frequently asked questions
What is a half-life?
The time for half of a substance to decay. After two half-lives a quarter remains, after three an eighth, and so on.
What's carbon-14's half-life?
About 5,730 years, which makes it useful for dating organic material up to roughly 50,000 years old.
Does this apply to medicines?
Yes — many drugs are eliminated with first-order kinetics; about 97% is gone after five half-lives.