A new method to determine the nip angle of a powder undergoing roller compaction process is proposed. This method is based on measuring the hardness of the pre-compacted body at different angular roll positions. The method was able to detect a change in the nip angle as a function of process parameters such as roll speed, which is not included in Johanson's theory, in addition to the effects of hydraulic pressure and roller gap. This study describes the compaction as a cumulative process consisting of many steps. The whole history of the powder compaction process was represented by the area under the curve of the Shore A hardness of the pre-compacted body at different angular roll positions. The area under the curve represents the progress of the compaction process. This information was found to be valuable for explaining changes in ribbon properties such as strength, porosity, width and outlet temperature in addition to the amount of fines (uncompacted powder) remaining during the roller compaction process. (C) 2016 Elsevier B.V. All rights reserved.