The settling velocities in water of 216 glass spheres ranging in size from 50mu to 5000mu in diameter were determined. The size, density, and shape of these spheres were accurately known. All timing precisions were better than 1/2% and the combined precision (size, shape, density, timing, etc.) of the velocity measurements was less than 2% at a 95% confidence level. Based on this data an empirical equation was derived to give the relationship between sphere size and settling velocity. The range of usefulness of the equation includes from 0.1mu to 6 mm. diameter spheres and, with correction factors, is extended to 50 mm. diameter spheres. Practical tables are presented for various sphere diameters, water temperatures, sphere densities, and fluid salinities. The equation and sphere data are proposed for use as a basis of standardization of settling tube data and as a basis for determining the sedimentation diameter.