Abstract Direct measurements of floc size and settling velocity are used to determine floc density distributions. In general, the results show that small flocs, containing relatively few primary particles, are highly compacted, while larger flocs exhibit densities which decrease rapidly with size. The density of the very largest flocs seems to level off at about the density of the original suspensions. Measurements have been carried out on well-characterized mineral suspensions containing particles of various sizes and mineral types flocculated under a variety of conditions. Based on a comparison of these results and on data from the literature, a generalized form for the floc density distribution is postulated.