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Femtocells that share the same spectrum with macrocells can enhance wireless coverage and system capacity. However, cross-tier interference between femtocells and macrocells complicates the process of two-tier network planning. In the network planning, interference margin (IM) is a key parameter for the link budget to indicate the impact of interference and calculate the cell coverage. The conventional IM evaluation method for orthogonal frequency division multiple access (OFDMA) systems does not consider the cross-tier interference and is unavailable for the co-existence of macrocell and femtocell scenario. In this paper, a proper method to obtain macrocell uplink IM considering the cross-tier interference is presented, and the closed-form expression of IM is derived as a function of system load. The simulation results confirm that the proposal can calculate the macrocell uplink IM accurately in two-tier OFDMA networks.