Radioactive Decay Calculator
Enter an isotope, initial activity, and elapsed time to calculate remaining activity, fraction remaining, and number of half-lives elapsed.
Use the Radioactive Decay Calculator
Decay Parameters
Decay Results
Enter parameters and click Calculate to see results.
Remaining Activity
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Fraction Remaining
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Half-Lives Elapsed
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n = t / t½
Decay Constant (λ)
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λ = ln(2) / t½ (per second)
Formula Used
A(t) = A₀ × 2^(−t / t½)
Summary
This tool applies the standard radioactive decay law (N(t) = N₀ · 2^(−t/t½)) to compute the remaining activity and quantity of a radioactive isotope after a given elapsed time. Select a preset isotope or enter a custom half-life, supply the initial activity and time elapsed, and the calculator instantly returns remaining activity, the fraction remaining, and the number of half-lives that have passed. Results compute instantly upon calculation with no server calls.
How it works
- Select a common isotope from the preset list or choose "Custom" to enter any half-life.
- Enter the initial activity (in Bq, kBq, MBq, GBq, or Ci) and elapsed time (in seconds, minutes, hours, days, or years).
- The calculator applies N(t) = N₀ × 2^(−t / t½) to compute the remaining activity.
- Results show remaining activity (in the same unit), the fraction and percentage remaining, and the number of half-lives elapsed.
- Use the "Copy Results" button to copy all values to your clipboard.
Use cases
- Verify radioisotope activity at time of use for medical physics and PET imaging.
- Plan safe handling and disposal schedules for radioactive waste.
- Calculate decay corrections for calibration sources in radiation detection.
- Support nuclear engineering coursework and exam preparation.
- Estimate remaining activity of industrial isotopes (e.g., Am-241 in smoke detectors).
- Cross-check decay chains in nuclear fuel cycle analysis.