A transfer function based calculation method is proposed to estimate radio frequency interference (RFI) problems. The closed-form equations are analytically derived from Maxwell's equations and the reciprocity theorem. The derived equations can clearly decompose the RFI problem into two parts: the noise source and the coupling transfer function to the antenna. Based on derivations, a transfer function concept is proposed to quantify the coupling coefficient from each unit dipole moment to the victim antenna. The transfer functions can be easily obtained from S-parameter measurements. The proposed method is validated through numeric simulations and real cellphone experiments. Engineering insights drawn from the method are also discussed. Overall, the proposed method is accurate, fast, and can provide physical insights for practical designs.