Aerosol particles at high concentrations grow by collisions and coalescence. If coalescence is faster, particles remain individual and spherical, and primary particle growth is determined by the collision rate. When collisions are faster, dendritic agglomerates consisting of a large number of primary particles form. Primary particle growth is then determined by the coalescence rate. In this Note a model is given for predicting primary particle growth based on the linear decay law for the surface area of a coalescing structure. In the collision-limited primary particle growth regime, the linear decay law is applied to the coalescing pairs of particles. In the coalescence-limited regime large agglomerates are present, and the linear decay law is applied to subregions of the agglomerates.