Donaldson R F Half-Life & Decay Calculator
How much of a radioactive (or any exponentially decaying) substance is left after a given time.
How this is worked out
Radioactive decay (and any other exponential decay, like drug elimination from the body) follows N(t) = N₀ × (½)t/T, where N₀ is the starting amount, t is elapsed time and T is the half-life — the time for exactly half of whatever remains to decay. Because the fraction remaining after one half-life is always exactly 50%, after two it's 25%, after three it's 12.5%, and so on, halving each time rather than losing a fixed amount.
Frequently asked questions
Does the substance ever completely disappear?
Mathematically, no — the exponential curve approaches zero but never quite reaches it, though after about 10 half-lives (under 0.1% remaining) the amount is usually considered negligible for practical purposes.
Can I use this for carbon dating?
The maths is the same (carbon-14's half-life is about 5,730 years, this tool's default), but real radiocarbon dating also corrects for things like historical variation in atmospheric carbon-14 levels, which this simple calculator doesn't attempt.
Related tools: Molar Mass Calculator, Molarity & Molality Calculator, pH & pOH Calculator.
Source / Last verified: standard exponential decay law (general chemistry/physics) — read/checked 1 October 2026. Carbon-14's commonly-cited half-life (5,730 years, the Cambridge half-life) is used only as this page's default example, not as a claim about a specific sample.