Formin FH2 domains associate processively with actin-filament barbed ends and modify their rate of growth. We modeled how the elongation rate depends on the concentrations of profilin and actin for four different formins. We assume that (1) FH2 domains are in rapid equilibrium among conformations that block or allow actin addition and that (2) profilin-actin is transferred rapidly to the barbed end from multiple profilin binding sites in formin FH1 domains. In agreement with previous experiments discussed below, we find an optimal profilin concentration with a maximal elongation rate that can exceed the rate of actin alone. High profilin concentrations suppress elongation, largely because free profilin displaces profilin-actin from FH1. The model supports a common polymerization mechanism for the four formin FH1FH2 constructs with differences attributed to varying parameter values. The mechanism does not require ATP hydrolysis by polymerized actin, but we cannot exclude that formins accelerate hydrolysis.