Half-Life Calculator

Pick what you need to find — solve for any one of the four decay variables.

Result
25

Half-life is the time it takes for a quantity to fall to exactly half its starting value — used for radioactive decay, drug elimination, and any process that decays at a rate proportional to what's left.

A worked example

Starting with 100g of a substance with a 5-year half-life, after 10 years (exactly two half-lives) only 25g remains.

Frequently asked questions

What kinds of things actually follow half-life decay?

Radioactive isotopes are the classic example, but the same math applies to drug elimination from the body, capacitor discharge in electronics, and any process where the rate of decay is proportional to the amount remaining.

Why doesn't the quantity ever hit exactly zero?

Exponential decay approaches zero but mathematically never quite reaches it — after enough half-lives, the remaining amount becomes negligibly small in practice, even though the formula technically keeps halving forever.