In this study, the roll damping coefficients of a high-speed planing hull are calculated numerically in order to show the effects of forward speed, roll amplitude and oscillation frequency. Forced roll simulations are carried out to calculate roll damping coefficients. The numerical study is performed at high forward speeds by using a commercial URANS (Unsteady Reynolds-averaged Navier-Stokes) solver. The uncertainty analysis of the numerical results is performed using the GCI (Grid Convergence Index) method in terms of grid spacing and time step size. The obtained numerical results are compared with existing experimental data and empirical methods. Results show that lift contribution is the major component of the roll damping at high speeds since the roll damping does not change considerably at different roll amplitudes and frequencies. It is also found that numerical calculations show better accuracy compared to the implemented empirical method.