There is a recent push to develop wild and non-domesticated Saccharomyces yeast strains into useful model systems for research in ecology and evolution, and for industrial and medical applications. Yet, the variation between species and strains in important population parameters, such as growth rate and carrying capacity, remains largely undescribed. Here, we investigated the relationship between two commonly used measures in microbiology to estimate growth rate – cell density and cell count - in 23 strains across all eight known Saccharomyces species with different ecological and geographic origins . We found that the slope of this relationship significantly differs among species. Thus, a given optical density (OD) does not translate into the same number of cells across species. We then speculated that this is due to species with smaller cells producing steeper slopes. While average cell size indeed differed between species, surprisingly, we found a slightly positive relationship between cell size and the slope of the cell density-cell count relationship, in the opposite direction than we predicted. Our results show that the strain- and species-specificity of the cell density and cell count relationship should be taken into account when running competition experiments requiring equal starting population sizes, when estimating the fitness of strains with different genetic backgrounds in experimental evolution studies, or when optimizing strains for industry. If we want to improve the biological interpretations of fitness data from wild yeasts, and draw meaningful conclusions from comparisons between wild and established laboratory strains, we need to calibrate our fitness estimates carefully.