A new guideline for the design of flocculators is proposed. This guideline is to design continuous flow flocculators on the basis of the characteristic reaction time for the loss of one micrometre particles in the suspension. The most commonly used guideline at the present time is that of Camp, but later proposals by Ives and O'Melia expanded the Camp guideline and incorporated more complete knowledge of flocculation. The suggestion here continues that process, by accounting explicitly for the heterodisperse nature of typical suspensions and the multiple collision mechanisms for particles in determining a characteristic reaction time. While the historical guidelines have served the water treatment industry well for reasonably similar size distributions in waters destabilized by precipitating metal hydroxides, the proposed guideline should be more robust in considering other suspensions. In particular, the new guideline can account explicitly for the effects of nanoparticles on flocculation.