Half-Life Calculator
Exponential decay, solved for the remaining amount, the half-life or the time.
The formula
N = N₀ × (1/2)t / T
Every half-life T cuts the amount in half: after two half-lives a quarter remains, after ten about 0.1%. Carbon-14 (T = 5,730 years) at 25% remaining means the sample is two half-lives, 11,460 years, old, which is exactly how radiocarbon dating works.
Beyond isotopes
The same math runs drug elimination (caffeine's half-life is about 5 hours, so a quarter of your morning coffee is still around at bedtime), charge draining from a capacitor, and any first-order process. The 'time units' are whatever you enter, as long as the half-life and elapsed time use the same one. Half-life describes populations, not individuals: one atom doesn't get 'half decayed', it just has a 50% chance per half-life.