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half-lifeClassic tool

Half-Life Calculator

Estimate the remaining amount after half-life decay and see the remaining percentage instantly.

Use this half-life calculator to estimate how much of a substance, signal or quantity remains after a given amount of time. It is useful for study, homework, radioactive decay, basic pharmacokinetics, chemical breakdown and any situation where reduction follows an exponential half-life pattern.

Enter the starting amount, the elapsed time and the length of one half-life using the same time unit for both time fields. The result shows the remaining amount, the amount lost, the remaining percentage and how many half-lives passed during the interval.

This is an ideal mathematical estimate. For medical, laboratory, environmental or safety decisions, use the output as a starting reference and confirm it with technical data, direct measurement or qualified guidance.

Use the same time unit for elapsed time and half-life.

Example: if the half-life is 3 hours and 12 hours have passed, 4 half-lives have elapsed.

SummaryEnter a starting amount, elapsed time and half-life to calculate the decay.
Remaining amount—
Amount lost—
Remaining percentage—
Lost percentage—
Elapsed half-lives—
Applied formula—

Use clear inputs to get a more useful result.