One of the main benefits of numerical simulations is the ability to shed light on the underlying physics in unit operations. This advantage was exploited in this work to study the die filling process in a lab-scale pharmaceutical rotary tablet press. The die filling process with emphasis on mass holdup, mass flow rate, tablet mass, and API content in the dies is described in detail. Eventually a design of experiments provides a clear picture on the influence of process conditions and material properties on the tablet mass to improve the process understanding.