Predictive microbiology is now an important tool in assessing the safe shelf life of foods. A draft tutorial illustrating how a Ratkowsky or square root model developed by Pan and Schaffner (2010) can be used to predict the growth of salmonella on tomatoes has been added to the DSFT website. The tutorial explains how the equation can be used and illustrates the use of freely available pathogen modelling programmes. A calculator programmed using the model can be used to explore the influence of temperature, initial numbers e.g. 1 CFU/1 kg and time on the growth of salmonella. The calculator was written in ASP and an explanation of how the coding works along with a free, downloadable zip file containing the scripts has been provided. Comments on how to improve the tutorial are welcomed.