Six of the most commonly used sphericity indices are examined to determine which is most suitable for predicting the settling velocity of nonspherical grains. Direct comparison is facilitated by normalizing the different measures for the same set of pebbles, which range in shape from spheroids, ellipsoids and disks, to blades. Regression analysis of the normalized sphericity indices plotted against the settling ratio (a modified dynamic shape factor) shows that the best index is the Hofmann (1994) shape entropy, followed by the sphericity measures of Aschenbrenner (1956) and Wadell (1932). The Corey (1949), Sneed and Folk (1958), and Janke (1966) shape factors are less suitable for a wide range of particle shapes. This is in contrast to some previous comparisons of the same indices.