Predicting the rock fragmentation obtained in drawpoints or secondary fragmentation is crucial in Block caving application since many engineering decisions are based on this key variable. These can include drawpoints size and spacing, equipment selection; draw control procedures, production rates, dilution entry and operational blasting requirements. Secondary fragmentation depends on several variables including structures, rock mass strength, the vertical pressure acting on the column, the rate of draw, and the height of the ore column. In order to study these variables, 18 experiments on gravity flow under confinement were run to quantify the fragmentation occurring in a draw column. Based on these experiments, a mathematical model was developed the basis of which considers a modified comminution model fitted using the experimental data. Finally, the fitted model was scaled up to represent what is expected to occur in practice compared to field data with an estimated accuracy of around 1.5% of size distributions. An approximation of the secondary fragmentation expected as a function of draw height and vertical pressure, for example for 400 m of ore column (mean vertical pressure of 4.2 MPa), the mean size, d(50), could decrease from 0.82 to 0.47 m and the large size, d(80), from 1.08 to 0.62 m.