For room acoustic simulations, the acoustic characteristics of room boundary surfaces are typically calculated under the assumption that the surfaces are sufficiently large. In this study, a reflection coefficient for finite surfaces is suggested and its performance is assessed through case studies involving simulation of rectangular rooms using a phased beam tracing method. The suggested reflection coefficient employs the radiation impedance of a finite surface by combining the radiation impedance of an infinitely large surface and that of the finite surface backed by an infinite rigid baffle. For 28 surface impedance values that are assigned to all the boundary surfaces, the suggested reflection coefficient is found to improve low frequency responses compared to the infinite panel theory; larger improvements are found for a more disproportionate room, more absorptive surfaces, and surfaces having larger negative phase angles of the surface impedance. A larger improvement is also found for a nonuniform absorption case than for a uniform absorption setting having a similar equivalent absorption coefficient.