The iodine-131 a doctor gives a thyroid patient is half gone in 8 days — hot enough to treat, short-lived enough to be safe within weeks. That predictable halving is the half-life: the time for half the atoms in any sample to decay. The remaining amount follows N = N₀ × (1/2)^(t / t½) — after one half-life half remains, after two a quarter, after three an eighth, marching down the powers of two.

Half-lives span the absurd. Francium-223 lasts 22 minutes; carbon-14 lasts 5,730 years, which is why archaeologists date ancient charcoal with it; uranium-238 lasts 4.5 billion years, which is why the Earth is still warm inside. Smoke detectors cheat beautifully with americium-241's 432-year half-life — so long the radiation barely changes over the device's life.

This calculator multiplies out the decay. Type the starting amount in the Variable A box — grams, atoms, or becquerels — and the remaining fraction (1/2)^(t/t½) in the Variable B box: 0.5 after one half-life, 0.25 after two, 0.125 after three. The Result box gives what is left. A 100-microcurie iodine dose is down to 12.5 microcuries after 24 days — three half-lives, three halvings.

With the pre-filled defaults — Variable A 10, Variable B 5 — the Half Life Calculator returns Result 50.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.