Where half-life applies
Half-life models radioactive decay and many first-order processes. Do not use it for systems where the rate changes with concentration, environment or biological feedback.
Example
If the initial amount is 100, half-life is 10 units of time and 30 units have elapsed, three half-lives have passed. The remaining amount is 100 x 0.5^3 = 12.5.
Common mistakes
Use the same time unit for half-life and elapsed time. A half-life in days cannot be mixed with elapsed time in hours unless you convert first.
Limitations
This calculator assumes a constant exponential decay rate. It is not a medical dosing tool or a substitute for specialist scientific modeling.
Last reviewed: 2026-05-17