The solid bed depth at the discharge end of rotary kilns was experimentally investigated for different mass flow rates. rotational speeds, inclination angles and materials using two lab kilns with sizes of 0.4 m (ID) * 5 m (L) and 0.25 m (ID) * 6.7 m (L). respectively. The solid depth at the discharge was found to be several more times higher than the particle diameter. All parameters according to Saeman's model were combined in a newly developed dimensionless 'Bed depth number' designated as 'Bd' The filling degree of a solid bed at the discharge can be correlated with F-0 = 1.75 * Bd(0.5) (for an inclination angle between 1 degrees and 4 degrees) The range of the researched Bed depth number (Bd) is suitable for all industrial kilns These values should be used as the initial condition, which was still unknown before, to solve the differential equation for the profile of the solid bed depth through the cylinder. (C) 2009 Published by Elsevier B V.