© 2020, Springer-Verlag GmbH Germany, part of Springer Nature. The interaction of reef-building coral and symbiotic dinoflagellates (family: Symbiodiniaceae) is of critical importance to organismal performance and ecosystem functioning. A standard biometric commonly used to evaluate coral physiology and health status, is coral biomass. Within-colony Symbiodiniaceae assemblages typically range from 1 to 6 million cells cm−2; however, no direct measurements have quantified the overall biomass of these dense symbiont populations. We use direct gravimetric measurements to determine the mass of individual Symbiodiniaceae cells and investigate the ratio of symbiotic dinoflagellate biomass to coral holobiont biomass in five species of scleractinian corals: Acropora cervicornis, A. palmata, Orbicella annularis O. faveolata, and Siderastrea radians from the Florida Keys, FL, USA (25.119°N, 80.302°W). Genetic identification of symbiotic dinoflagellates revealed a single, dominant population of symbiotic dinoflagellates among each scleractinian species, suggesting that the corals did not change symbionts during this study. The overall contribution of the symbionts to holobiont biomass ranged from ~ 5 to 15%, and remained relatively constant throughout the winter and summer for all species with the exception of the Acroporids. This relatively small biomass of symbionts is in contrast to the millions of cells cm−2 which are present in these corals, and that enhance calcification, recycle nutrients, and supply nutrition to their reef-building hosts.