Some studies have proposed the estimation of convective rainfall from lightning observations by the computation of the rainfall–lightning ratio (RLR). However, as such ratio may depend on season, convective regime and other factors, known approaches failed to provide values of RLR with low variability. An accurate RLR would allow estimating rainfall from lightning data in areas that lack weather radar coverage. This work proposes a straightforward approach for the computation of RLR, based on a temporal sliding-window and a fitting function. It was tested for thunderstorms observed in the Southeastern Brazil with good results.