Modification of viscous drag and the pressure drag terms in Rubey's theoretic equation for settling velocity of sand grains with two empirically determined constants results in the following formula urn:x-wiley:00431397:media:wrcr583:wrcr583-math-0001 which predicts accurately the setting velocity of groups of sand grains measured with the Woods Hole rapid sediment analyzer. The following equation is a solution of Rubey's modified general equation that can be used to calculate the diameter of a particle or group of particles if the settling velocity for the particle is known: urn:x-wiley:00431397:media:wrcr583:wrcr583-math-0002