The determination of plant tissue volume may be used to estimate the fresh weight of calli obtained in vitro; however, the use of techniques for the indirect measurement of callus volume in the in vitro multiplication of cacti has not been reported widely. The non-destructive indirect measurement of growth variables during in vitro propagation is very useful in the massive multiplication of plant species in danger of extinction, as is the case of Strombocactus disciformis (Britton & Rose 1922), a Cactaceae of the arid and semi-arid zones of Mexico. This study analysed six regression models in order to obtain a mathematical expression to estimate the volume of fresh calli of Strombocactus disciformis, based on non-destructive measurements. A linear relationship was observed among the variables of the study and, based on a selection criterium, the model corresponding to a flattened semi-sphere, reduced to one variable, was the best model to estimate the volume and fresh weight of the plant tissue.