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Radioactive Decay (Becquerel) Calculator

Estimate the activity (Bq) left after radioactive decay from an initial becquerel value, nuclide, and elapsed time. Shows the remaining fraction and amount decayed too.

Input

Radioactive substances decay at a constant rate over time, so their activity (becquerel value) steadily decreases. Enter the initial activity, nuclide, and elapsed time to estimate the activity remaining after decay.

Bq
Nuclide (radioactive substance)
Elapsed time unit

Result

Activity after decay (estimate)

74.81Bq

Iodine-131 (I-131): decayed from an initial1,000 Bq

Remaining fraction

7.48 %

Nuclide half-life

8.02 days

Amount decayed

925.19 Bq

Half-lives elapsed

3.74 half-lives

Half-life in days

8 days

This estimate uses the radioactive decay formula A = A0 × (1/2)^(t/T). Actual values in food or the environment vary due to mixed nuclides and measurement error.

Remaining fraction per half-life (rough guide)

Half-lives elapsedRemaining fraction
1 half-lives50 %
2 half-lives25 %
3 half-lives12.5 %
4 half-lives6.25 %
5 half-lives3.13 %
10 half-lives0.1 %

How it works

  • The remaining activity is calculated with the radioactive decay formula A = A0 × (1/2)^(t/T), where A0 is the initial activity, T is the half-life, and t is the elapsed time.
  • Activity falls by roughly half with each half-life. After 2 half-lives about 25% remains, after 3 about 12.5%, and after 10 about 0.1%.
  • Choosing a nuclide automatically sets a representative half-life (for example, Iodine-131 about 8.02 days, Cesium-134 about 2.06 years, Cesium-137 about 30.17 years). For substances not listed, enter the half-life directly.
  • The half-life and the elapsed time can use different units (days or years). Internally 1 year is treated as 365.25 days.
  • This computes only the decrease in activity from the physical half-life. It does not account for biological elimination (biological half-life), mixed nuclides, or measurement error.
  • Any judgment about radiation or radioactivity is only a rough guide. For actual values and safety, always refer to official information published by public authorities.