Effective Half-Life Calculator
Calculate the effective half-life — how long it actually takes for a radioactive substance in the body to halve — from its physical and biological half-lives.
Input
A radioactive substance taken into the body decreases through both radioactive decay (physical half-life) and metabolism and excretion (biological half-life). Enter both in the same unit to calculate the effective half-life — the time it actually takes for the amount in the body to halve.
Result
Effective half-life Te (approx.)
7.27days
Physical half-life Tp
8 days
Biological half-life Tb
80 days
Effective half-life Te
7.27 days
Estimated as Te = (Tp × Tb) / (Tp + Tb). The effective half-life is always shorter than either the physical or the biological half-life.
Calculation breakdown
| Item | Value |
|---|---|
| Physical half-life Tp | 8 days |
| Biological half-life Tb | 80 days |
| Tp + Tb | 88 days |
| Effective half-life Te | 7.27 days |
This tool gives an estimate based on a well-established physical formula. Actual biokinetics vary by substance and individual, so consult official sources for specialized judgments such as exposure assessments.
How it works
- A radioactive substance taken into the body decreases at the same time through two mechanisms: reduction by radioactive decay (physical half-life Tp) and removal from the body by metabolism and excretion (biological half-life Tb).
- Assuming the two decays act independently, their rates add up, so the effective half-life Te is given by 1/Te = 1/Tp + 1/Tb, which solves to Te = (Tp × Tb) / (Tp + Tb).
- The effective half-life Te is always shorter than either the physical half-life Tp or the biological half-life Tb, because the two effects combine to reduce the amount in the body more quickly.
- Always enter the physical half-life Tp and the biological half-life Tb in the same time unit (days, years, etc.). The resulting Te is shown in that same unit.
- The biological half-life is an approximate value that varies greatly with the type of substance, route of intake, age, and metabolism. For actual exposure assessments, do not rely on this estimate alone — refer to the latest information from official bodies or to expert judgment.
- This tool provides an estimate based on an established physical formula and is not a substitute for a formal medical or radiation-protection assessment.
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Effective Half-Life Calculator