Design specifications for solids suspension (just suspended speed-N-js) in stirred tanks are currently based on the original form of the Zwietering correlation. There are several limitations of this correlation which can lead to poor predictions. The limitations, however, are not clearly defined. In this paper we review the reported limitations of applicability of the Zwietering correlation in detail, and propose ways to predict N-js which extend some of these limits. New data is reported and a detailed analysis of the effects of off-bottom clearance and solids concentration is presented. These results show two things: first that the effect of off-bottom clearance cannot be explicitly accounted for using the Zwietering equation form. S values that exactly match the geometry of the system must be used. Second, the current exponent on solids concentration is applicable only up to 2 wt% solids (X = 2). Three new exponents are proposed based on the particle type: 0.17, 0.23, and 0.32. Using a modified exponent of 0.23, which represents the entire data set with some scatter, N-js can be predicted up to 35 wt% solids (X = 54). A new correlation that is a combination of Zwietering and Baldi, and is robust in scale-up based on constant power/mass is proposed. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.