In all of the above examples, the initial nuclide decays into just one product. Consider the case of one initial nuclide that can decay into either of two products, that is  and  in parallel. For example, in a sample of potassium-40, 89.3% of the nuclei decay to calcium-40 and 10.7% to argon-40. We have for all time :
which is constant, since the total number of nuclides remains constant. Differentiating with respect to time:
defining the total decay constant  in terms of the sum of partial decay constants  and :
Solving this equation for :
where  is the initial number of nuclide A. When measuring the production of one nuclide, one can only observe the total decay constant . The decay constants  and  determine the probability for the decay to result in products  or  as follows:
because the fraction  of nuclei decay into  while the fraction  of nuclei decay into .