A quantity decay model for which the rate of decay is directly proportional to the amount present. The equation for the model is A = A₀b^t (where 0 < b < 1 ) or A = A₀e^kt (where k is a negative number representing the rate of decay). In both formulas A0 is the original amount present at time t = 0.

This model is used for phenomena such as radioactivity or depreciation. For example, A = 50e^–0.01t is a model for exponential decay of 50 grams of a radioactive element that decays at a rate of 1% per year.