A banana is radioactive, your smoke detector is radioactive — the difference is only the half-life. Radioactive decay follows one elegant rule: after each half-life, half of what remains is gone, so the amount left equals the starting amount times one-half raised to the number of half-lives elapsed, N = N₀ × (1/2)^(t/T). Ten half-lives leave about a thousandth; twenty leave about a millionth. The atoms do not age or slow down — each nucleus simply rolls the same dice, forever.

This calculator applies the surviving fraction. Type the starting quantity into Variable A — atoms, grams, or becquerels, any unit works — and the remaining fraction (1/2)^(t/T) for your elapsed time into Variable B; the Result box shows what is left. A nuclear-medicine tech in Houston drawing a technetium-99m dose, for instance, enters the 6-hour half-life fraction for the hours until injection and reads the usable activity straight off. Carbon dating runs the same equation backward: measure what remains, and the fraction reveals how many half-lives — 5,730 years each for carbon-14 — have passed.

With the pre-filled defaults — Variable A 10, Variable B 5 — the Radioactive 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.