Laboratory laser Doppler velocimetry (LDV) measurements were conducted to measure the shear rate coefficient K-s of a range of impellers. Equations correlating Ks with N-Q (flow number) are provided for axial flow and radial flow impellers. Theoretical formulations based on the classic boundary layer theory are developed to estimate the shear rate at the blade surface. Calculations show that the shear rates at the blade surface are many orders of magnitude higher than those in the flow at the impeller outlet. The software code XFOIL was used to illustrate typical distributions of the shear rates along the blade surfaces. Effects of viscosity, non-Newtonian shear-thinning index, agitator design, and scale-up on shear rates are illustrated. (c) 2006 American Institute of Chemical Engineers.