The loading state of a rotary drum strongly affects its overall performance. Hence, assessing the optimum (design) loading of the rotary drum is a critical issue. In the present work, a lot of experiments were carried out on a 50 cm diameter and 15 cm length batch rotary drum furnished internally with rectangular flights. Different loadings were examined through image analysis processing of the recorded videos from under-loading to over-loading, including design-loading. Many solid materials (free flowing) with different particle diameters and angles of repose, varied rotational speeds (from 1 to 5 rpm), two numbers of flights (12 and 18), and two flight length ratios (0.375 and 0.75) were researched. A comparison is conducted between the experimental results and some available design loading models from literature. Based on the experimental results a new fitting factor is proposed. (C) 2015 Elsevier Ltd. All rights reserved.