Simple but reliable correlations for a drag coefficient, C_D, of single bubbles under a wide range of fluid properties, bubble diameter and acceleration of gravity were developed based on a balance of forces acting on a bubble in a stagnant liquid and available empirical correlations of terminal rising velocities of single bubbles. The proposed C_D consists of three equations, each of which corresponds to pure, slightly contaminated and contaminated systems. The effect of a frictional pressure gradient due to a liquid flow is also taken into account by introducing a concept of an effective body acceleration. Terminal rising velocities of single bubbles were calculated using the proposed C_D, and compared with measured data under the condition of 10^-2<Eo<10³, 10^-14<M<10⁷ and 10^-3<Re<10⁵ where Eo, M and Re are Eotvos, Morton and bubble Reynolds numbers, respectively. As a result, it was confirmed that the proposed C_D gives better predictions than available drag coefficient models.