Half-Life Calculator

Find the amount left after a time, the time to reach a target fraction and the number of half-lives.

Radioactive decay

Initial amount or activity Half-life Time unit

Elapsed time Target fraction left

Remaining amount: —

Fraction remaining: —

Half-lives elapsed: —

Time to reach the target fraction: —

Decay constant: —

—

Both the amount and the activity follow the same decay curve, halving once every half-life.

Common questions about the free online half-life calculator.

Enter the starting amount, the half-life and the elapsed time, and the tool returns the remaining amount and the fraction still left.

The remaining amount is the starting amount multiplied by 0.5 raised to the number of half-lives that have already passed.

Divide the elapsed time by the half-life. Two half-lives leave one quarter and three half-lives leave one eighth of the original.

The tool uses the half-life times the natural logarithm of 1 divided by the target, all divided by the logarithm of 2.

The decay constant is the natural logarithm of 2 divided by the half-life, giving the fraction that decays in one unit of time.

The table lists each whole half-life with the elapsed time, the remaining amount and the percentage of the original still present.

Yes. Pick seconds, minutes, hours, days or years and both the half-life and the elapsed time are read in that same unit.

No. Any quantity that halves, such as mass, atom count or counts per second, follows exactly the same decay curve.

With no time elapsed nothing has decayed, so the remaining amount equals the starting amount and the fraction left is 1.

No, the half-life calculator runs entirely in your browser, needs no account and stores nothing.

Last updated: October 10, 2026