In this report the value of the shape factor of quarry rock is reassessed. This parameter is defined as the ratio of the median nominal diameter and median sieve diameter of quarry rock gradings. Both diameters describe 'the size' of quarry rock and are commonly applied in different phases throughout hydraulic engineering practice. Since these phases overlap, it is necessary to enable the interchange of both diameters, which explains the importance of the value of the shape factor. By a researcher named G.J. Laan in the 1980's, the value of the shape factor was found to be 0.84. Unfortunately Laan passed away and the specific report is missing from the libraries. Despite the resulting lack of theoretical background the value of 0.84 is still widely applied in hydraulic engineering practice. By writing my master thesis I hope to have taken away this lack of theoretical background.