Half-Life Calculator
Calculate the remaining amount of a substance or the elapsed time using radioactive decay.
Assumptions used in this calculation
- Units must match: Elapsed time and half-life must be in the same time unit for the result to be meaningful.
About this calculator
Radioactive decay and other exponential decay processes (drug elimination from the bloodstream, capacitor discharge) don't proceed at a constant rate, so you can't just divide the remaining amount proportionally by time, a substance loses half of whatever remains during each successive half-life, not a fixed absolute amount, which makes manual estimation unreliable without applying the exponential formula correctly. This calculator solves N(t) = N₀ × 0.5^(t ÷ half-life) for either the remaining amount after a given elapsed time, or the elapsed time needed to reach a given remaining amount, given the initial quantity and the substance's known half-life. It's useful for radiometric dating problems, pharmacology calculations involving drug half-lives, nuclear chemistry coursework, or estimating how long a radioactive sample needs to be stored before it decays to a safe level, without manually working through repeated halving or logarithms by hand.
Worked example
N₀=100, t=15, half-life=5
Result: 12.5 remaining (12.5%)
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