A quantity is subject to exponential decay if it decreases at a rate proportional to its current value. The decay law calculates the number of undecayed nuclei in a given radioactive substance.

Decay Formula – 

Formula for Half-Life in Exponential Decay –

\[\large N(t)=N_{0}\left ( \frac{1}{2}^{\frac{t}{t_{\frac{1}{2}}}} \right )\]

\[\large N(t)=N_{0}e^{\frac{-t}{\tau }}\]

\[\large N(t)=N_{0}e^-\lambda t\]

is the initial quantity of the substance that will decay (this quantity may be measured in grams, moles, number of atoms, etc.),
N(t) is the quantity that still remains and has not yet decayed after a time t,
is the half-life of the decaying quantity,
is a positive number called the mean lifetime of the decaying quantity,

is a positive number called the decay constant of the decaying quantity.

The three parameters

, , and are all directly related:

\[\large t_{\frac{1}{2}}=\frac{\ln (2)}{\lambda}=\tau  \ln(2)\]