A new method to predict the size of the air-core within a hydrocyclone is presented based on calculating the internal pressure distribution by solving a set of conservation equations. The effect of slurry properties such as bulk viscosity and surface tension and flow conditions are taken into consideration. Predictions of air-core diameters are given as a function of various hydrocyclone geometries and operational conditions. The results show that feed flow rate has a dominant effect on the air-core diameter. The results predict a slightly conically shaped air-core with the cone-angle depending on the the slurry viscosity. Increasing the slurry viscosity causes a decrease in the air-core diameter.