This report describes a quantitative method for calculating overdesign factors for coal preparation plant design. A Monte Carlo technique was used to account for uncertainties in design data and plant operation, and a statistical consideration of possible plant performance enables the selection of an appropriate overdesign factor with respect to the required level of confidence in the design. The uncertain design factors considered were percentage of fines, coarse coal yield, fine coal yield, amount of out-of-seam dilution and availability. In general, the justifiable overdesign factor increased with increasing uncertainty in data and with a greater number of variables in which there was uncertainty. The variability of parameters which had a large impact on overall yield contributed significantly to overdesign factors. From this study the most important variables for determining overdesign factors were uncertainties in coarse coal yield and plant availability with the variability in out-of-seam dilution having a lesser but still important effect. Although aspects of fine coal treatment had little impact on overall overdesign, the design capacity of the fines plant should be considered separately. The required degree of confidence in plant design has a strong influence on the over-design factor, with a steady increase in overdesign factor up to 90% confidence, but a very rapid increase above 90%