A method and system for modeling growth of reductant deposits for an aftertreatment system on a real time basis using input including exhaust gas temperature, exhaust gas flow rate, and reductant dosing rate. The growth of the reductant deposits are affected by a rate at which reductant accumulates and decomposes from a surface of the aftertreatment system. Thus, the method and system disclosed determines a net reductant deposit growth rate value based on a reductant deposit accumulation rate value and a reductant deposit decomposition rate value. Further, the method and system disclosed determines a reductant mass deposit value based on the net reductant deposit growth rate value. The reductant mass deposit value determined by the method and system may then be used to control a regeneration strategy of the aftertreatment system to eliminate the reductant deposits from the aftertreatment system.