The previous analytical solution for the drag coefficient (Cd) for a spherical particle attached on the flat surface, which was derived by O’Neill (1968), is only valid in the creeping flow conditions. It is important to extend O’Neill’s formula to cover a wide range of particle Reynolds number (Rep). In this study, the drag coefficient was calculated numerically to cover Rep from 0.1 to 250. For a particle suspended in the air, an empirical drag coefficient exists, which is defined as Cd = f × 24/Rep, where f is a correction factor depending on Rep. The applicability of the correction factor f for O’Neill’s analytical equation for the spherical particle attached on the flat surface for Rep = 0.1 to 250 was examined in this study.