The rheological behaviour of Newtonian suspensions of platelets with an aspect ratio ranging from 0.006 to 0.2 is studied in this work. Using model particles to correlate the aspect ratios square-shaped particles with their sedimentation behaviour, the results obtained were used to estimate the average aspect ratio of mica particles. A comparison is made between the aspect ratio measured by sedimentation experiments and that determined from the rheological behaviour of dilute suspensions. Their good agreement shows that the average aspect ratio of a polydispersed suspension of platelets estimated using packing experiments correlates well with the apparent aspect ratio given by the intrinsic viscosity of the suspension.