Half-Life Calculator
Solve remaining amount, elapsed time or half-life for a constant first-order exponential decay model.
What would you like to do next?
Solve three forms of a first-order half-life model
Select remaining amount, elapsed time or half-life as the unknown, then enter the values that define the other parts of the decay. The calculator also reports how many half-lives have elapsed. This makes it possible to check repeated-halving intuition against a continuous exponential result, including cases that do not end on a whole number of half-lives.
Confirm that a constant first-order model applies
Radioactive decay is a familiar use, but not every chemical, biological or reliability process follows one stable half-life. Multiple populations, changing environments, background signals and other reaction orders can produce different curves. Treat units consistently, preserve measurement uncertainty and fit an appropriate model before extrapolating far beyond observed data.
Frequently asked questions
Must elapsed time be a whole number of half-lives?
No. The exponential relationship supports fractional half-life counts.
Can the tool calculate elapsed time?
Yes. Choose elapsed time and enter initial amount, remaining amount and half-life.
Does every decay process use this equation?
No. It assumes one constant first-order decay rate.