Pick your radioisotope (or enter a custom half-life) alongside an initial activity and a decay time, and this Radioactive Decay Calculator tells you the final activity remaining — plus the decay percentage, remaining activity, and exactly how many half-lives have elapsed.
Final Activity
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Decay Percentage
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Remaining Activity
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Half-Lives Elapsed
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Half-Life Used
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Radioactive decay is the spontaneous process by which unstable atomic nuclei lose energy by emitting radiation. The rate of decay is characterized by the half-life, which is the time it takes for half of the radioactive atoms to decay.
Radioactive decay follows an exponential decay formula: A(t) = A₀ × e^(-λt), where A(t) is the activity at time t, A₀ is the initial activity, λ is the decay constant, and t is time. The decay constant is related to half-life by λ = ln(2)/t₁/₂.
Both measure radioactivity but in different systems. The Curie (Ci) is the traditional unit where 1 Ci = 3.7 × 10¹⁰ disintegrations per second. The Becquerel (Bq) is the SI unit where 1 Bq = 1 disintegration per second. 1 Ci = 37 GBq.
The decay rate is only determined by the half-life of the specific isotope and is independent of external conditions like temperature, pressure, or chemical form. Each isotope has a fixed half-life that cannot be changed.
Decay calculations are very accurate for large numbers of atoms using the statistical decay law. For small samples, there may be statistical fluctuations. The calculations assume pure isotopes and don't account for measurement uncertainties.
Half-life determines how long radioactive materials remain hazardous. Materials with short half-lives decay quickly but are initially very radioactive, while long half-life materials remain radioactive for extended periods but at lower levels.
No, radioactive decay cannot be stopped, slowed down, or accelerated by any chemical or physical process under normal conditions. The decay rate is an intrinsic property of the atomic nucleus and follows precise statistical laws.