Nuclides
About the app ↗
Free web tool · Single-nuclide model
Radioactive decay calculator
Calculate remaining activity, work backwards to the initial activity, or find the time needed to reach a lower target. Select a nuclide or enter a known half-life.
Calculation inputs
Choose a nuclide from the reference
Examples: Cs-137, 137Cs, Tc-99m, 99mTc, cobalt, or Z=27. Neutron systems use n-4 / n-6.
Loading basic reference data…
Custom half-life
Decay result
Elapsed durations use 365.25 days per year. Date comparisons use UTC, with no daylight-saving adjustment.
How to calculate radioactive decay
A(t) = A₀ × exp(−λt) · λ = ln(2) / T½
After one half-life, 50% of the initial activity remains. After two half-lives, 25% remains. To work backwards, A₀ = A(t) × exp(λt). To find a decay interval, t = ln(A₀ / A(t)) / λ.
This model describes the physical decay of one isolated radionuclide. It excludes parent–daughter ingrowth, radioactive equilibrium, continuing production, biological clearance and measurement uncertainty. It does not determine a clinical dose or establish a safe handling or release time.
Half-lives are taken from the Nuclides basic reference snapshot. Values with limits and records expressed as energy widths are displayed in lookup but are not selectable for a point calculation. Approximate values retain an approximation notice. You can enter a custom half-life from your own validated source.
Method: US NRC: half-life. Calculation inputs stay in your browser.
Take your reference with you
Keep Nuclides on your phone.
Explore decay chains, radiation energies and connected references in the app. Bundled reference data works offline; online resources need a connection. Check each store for pricing and in-app purchases.
