Breakthrough-curve (BC) predictions for newly regulated per- and polyfluoroalkyl substances (PFAS) using granular activated carbon (GAC) are needed by drinking water engineers during the conceptual design process. Conceptual designs must commonly be developed without extensive water quality data and bench- or pilot-scale testing. Therefore, a BC modeling approach was developed using the literature Freundlich isotherm values, pore and surface diffusion models, and apparent adsorption capacity parameter (K*) to inform these design steps and potentially lower costs through testing avoidance. Predicted BCs for perfluorooctanesulfonic acid (PFOS), the most strongly adsorbing regulated PFAS, were developed for typical groundwater treatment scenarios related to intraparticle diffusion, influent concentration, empty bed contact time, dissolved organic matter fouling, and adsorber design. On average, predicted PFOS BCs were not significantly different from observed PFOS BCs obtained from the literature when influent PFOS concentrations were greater than its new maximum contaminant level of 4 ng/L. PFOS BCs were then scaled using PFAS-specific adjustment factors developed from literature data and applied to PFOS K* values to generate other regulated PFAS BCs. Such an approach is likely applicable to other groundwater organic contaminants and could be modified for predicting the removal of PFAS and other organic contaminants from surface water.