Radioactive Decay Calculator

Your details

Choose which quantity to calculate. The others become inputs.
Pick a common isotope to auto-fill the half-life. Choose "Custom" to enter your own.
Starting quantity, in grams, atoms, moles, becquerels, or any consistent unit.
units
Select the time unit for half-life and elapsed time. Both must use the same unit.
How long the sample has been decaying, in the same time unit as the half-life.
y
Turn on to calculate the radioactive activity in becquerels and curies. Requires the sample mass and molar mass.
Amount remainingSubstantially decayed
24.93units
Percent remaining24.93%
Amount decayed75.07units
Half-lives elapsed2.004
Decay constant (λ)0.000121/ time unit
Mean lifetime (τ)8,249.954time units
24.93% %
Nearly gone<1Mostly decayed1-10Substantially decayed10-50Most remains50+

After 2 half-lives, 24.93% of the sample remains.

  • After 2 half-lives, 24.93% of the sample remains (24.93 out of 100 original units).
  • Each half-life removes exactly half of whatever is left, so the surviving fraction is (1/2)^2.004.
  • The decay constant λ = 1.21e-4 per time unit; multiply by the number of atoms for the instantaneous activity.

Next stepSwitch the "Solve for" dropdown to "Elapsed time" or "Initial amount" to reverse-solve the decay equation.

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