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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.

Time unit

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

ItemValue
Physical half-life Tp8 days
Biological half-life Tb80 days
Tp + Tb88 days
Effective half-life Te7.27 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.