Within this study, the Newton/Reynolds characteristics of a continuous rotor/stator mixer for foam production were determined with special geometrically adapted Newton and Reynolds numbers. A critical Reynolds number of 105 was derived. Equations for the laminar and turbulent flow field for pre-calculating the maximum bubble size in rotor/stater mixers were derived and experimentally confirmed. These equations combine process, recipe and geometry parameters and a direct scale-up is possible. An increase of the rotor speed from 600 to 2000 rpm resulted in a decrease in bubble size of about a factor of 4.5 for a low viscous liquid, whereas for a highly viscous liquid, the decrease in bubble size in the same rotor speed range was only 20%. Drainage stability improvement of about 100% and an increase in foam viscosity of about 100% were achieved in the same rpm range.