The goal of this work was demonstrating the zootechnical indices calculations and their applications in a methodology for colonies sizing, aiming at the adjustment of production. For this, 120 breeders of Wistar rats were evaluated, with data collection carried out between January and December 2021. The information recorded were: dates of birth of the male and female, date of mating and parturition, number of pups/parturition, number of male and female weaned/parturition, number of euthanized animals. The zootechnical indices (IZ) calculated were: time to first parturition, interval between parturitions (IEP), culling rate, monthly parturition index (IMP), number of animals weaned/parturition, weaning rate, proportion of male and females weaned/parturition, pre-weaning mortality and reproductive lifespan. Descriptive statistics were performed and when necessary, ANOVA was performed followed by Tukey’s post-test with a significance of p<0.05. Based on IZ information, a methodology was proposed to calculate the number of breeders. To size the colony, the first step was calculating the number of parturitions, i.e., dividing the number of animals demanded by the number of pups weaned/parturition. The second step consisted in calculating a correction factor, dividing the IEP by 30 and multiplying by the IMP, applying this, the time required for the breeder to have new offspring and the percentage of breeders that gave birth monthly were considered in the calculus. Finally, to calculate the number of breeders, just divide the number of births by the correction factor. The systematization of the IZ enables to consider important colony characteristics for sizing production, optimizing the use of resources and reducing over production.