It is commonly assumed in analyses of grain settling, particle threshold, evaluations of sphericity and in measurements of gravel fabric that particles can be approximated by regular triaxial ellipsoids. This assumption is explored for several data sets including sediment particles ranging in size from sand to pebbles (0.1 mm to 5.5 cm). The true nominal diameter, D n , calculated from the measured weight or volume, is compared with the nominal diameter of a geometric ellipsoid, D ne , calculated from its three measured axial diameters. There is a close correspondence for all size ranges, the best agreement being for regular pebbles selected on the basis of their visual similarity with ellipsoids. Grain irregularities cause D ne to be less than D n , but even with highly irregular natural sand grains the correspondence remains good. The nominal diameter is also compared with other size measures, including the average of the three axial diameters and the intermediate diameter alone, yielding empirical relationships for the prediction of this important size parameter.