The storage capacity and rate of rise of a tailings storage facility (TSF) constructed utilising the tailings to form the TSF is dependent on the outer side slopes of the facility. A number of TSFs in South Africa are operated utilising cyclones to construct the outer wall zone of the TSF. The outer slope is dependent on the stacking angle of the cyclone underflow and hence it is critical that the design of the TSF accurately estimates the stacking angle. There are no clear guidelines to make this estimation other than to compare to similar projects. This methodology has resulted in a few TSFs in which the stacking angle was either over or underestimated leading to an inaccurate life assessment. This paper hypothesises that the yield stress of the underflow has the most significant effect on the stacking angle. The hypothesis is tested by measuring various tailings properties on a tailings operation in relation to the stacking angle achieved to attempt to develop a method to predict the stacking angle for future projects.