Half-Life Calculator

Enter an initial amount, half-life, and elapsed time to calculate how much remains.

How to Use the Half-Life Calculator

Elapsed time is what actually drives the decay curve here — try stretching it well past a single half-life to see the amount keep halving indefinitely rather than hitting zero.

remaining = initial amount × 0.5^(elapsed time ÷ half-life)

One half-life leaves half. Two leaves a quarter. Three leaves an eighth — each additional half-life cuts whatever's left in half again, which is also why the remaining amount technically never hits zero: halving a positive number always leaves a smaller positive number, forever, even once it's negligibly small in practice. This Half-Life Calculator works out how much remains after any elapsed time.

For radioactive decay specifically, half-life is a fixed property of the isotope itself — indifferent to temperature, pressure, or chemical state, and unaffected by how much material you started with. Different isotopes range from a fraction of a second to billions of years, but whichever one you're dealing with, its half-life doesn't budge under normal conditions.