Fluctuation analyses is the standard experimental method for measuring mutation rates in microorganisms. The appearance of mutants is classically described by a Luria-Delbrück distribution composed of two parameters: the number of mutations per culture (m) and the differential growth rate between mutant and wild-type cells (b). A precise estimation of these 2 parameters is a prerequisite to the calculation of the mutation rate. Here, we developed bz-rates, a web-tool to calculate mutation rates that provides three useful advances over existing web-tools. First, it allows taking into account b, the differential growth rate between mutant and wild-type cells, in the estimation of m with the Generating Function (GF). Secondly, bz-rates allows the user to take into account a deviation from the Luria-Delbrück distribution called z, the plating efficiency, in the estimation of m. Finally, the web-site provides a graphical visualization of the goodness-of-fit between the experimental data and the model.