Abstract: In this study, we present a new method of calculating the reactivity using the inverse point kinetic equation. We discretize the history of the neutron population density with the integration method known as Simpson’s 3/8. We resolve it numerically with the sum of a convolution. For this study we develop diverse numerical experiments, in which we consider different expressions for the neutron population density and different time steps. When we compare the results acquired with some methods reported in the literature, we find that the proposed method gives correct results and therefore can be used to calculate reactivity correctly.