Rheological behaviour of fluids in mixing systems was studied using a simplified approach in shear rate calculation. The efficiency of this approach is investigated from Newtonian and non-Newtonian fluids, exploring also the geometry of mixing systems. Results are favourably compared with those obtained from previous published works. It is shown that this approach does not require the calculation of a virtual inner radius nor a priori assumption of the rheological behaviour of the fluid mixed. (c) 2008 Elsevier Ltd. All rights reserved.