The estimation of the earthquake size distribution parameters is one of the most important parts in any seismic hazard study. GR_EST toolbox is a source code written for OCTAVE/MATLAB (Eaton et al., 2019; MATLAB, 2019) that allows estimating these parameters in a proper way, including the estimation of the associated uncertainties. The toolbox contains functions to make the parameter estimation both for instrumental and historical seismic catalogs, also considering time‐varying completeness for magnitudes. Different functional forms for the magnitude–frequency distribution and different strategies for the estimation of its parameters and relative uncertainty are included. To guide the seismologists into the use of this toolbox, a set of complete examples is provided, to be used as “how to” use cases.