Quarry rock is a frequently applied construction material in hydraulic engineering. The main reason is its unique stability capacities in environments of high hydrodynamic activity. Furthermore, quarry rock is produced and handled relatively easily and available at numerous locations across the globe. The size, or more precisely, the dimension of rock plays an important role in the realization of granular hydraulic structures. In general, two definitions are applied for the dimension of quarry rock, viz.: the nominal diameter and the sieve diameter. The ratio of both diameters is referred to as the shape factor of quarry rock and is frequently applied to express the one diameter in the other. Quarry rock however consists of a practically infinite variety of sizes and shapes and as a result the material is typically graded. For quarry gradings therefore the shape factor is based on ‘median’ values for the nominal diameter and sieve diameter, which are related to the fifty percent cumulative mass percentage of the grading. In a report published by G.J. Laan (1981) the value of the shape factor for quarry rock was determined to be 0.84. Unfortunately, Laan passed away and the concerned 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, as is recommended by - amongst other literature - the Rock Manual (2007). In this thesis the value of the shape factor has been reassessed. By conducting experiments on 21 samples, consisting of 200-250 rocks, extracted from various gradings, it is found that the shape factor is reasonably well represented by means of a Gaussian distribution. The average value and standard deviation are 0.86 and 0.24 respectively. Furthermore, it is concluded that the value of the shape factor is not constant within a grading. The value of the shape factor slightly decreases for increasing rock size. Such relation is however not discerned over the coarseness of the tested gradings. According to the tested gradings, the value of the shape factor is independent from the shape of the rock, in terms of both elongation and blockiness. For individual rock however, on average the value of the shape factor slightly increases for an increase of elongation as well as blockiness. As a consequence it cannot be completely ruled out that, for gradings consisting of a significantly higher percentage of relatively elongated or blocky rock compared with the tested gradings, the shape factor will increase. Like the shape factor for rock gradings, the shape factor of individual rocks is also represented reasonably well by means of a Gaussian distribution. The average value and standard deviation are 0.84 and 0.083 respectively. Moreover, it is found that the shape factor of an individual rock may exceed the value of 1.0. For such rock the nominal diameter exceeds the sieve diameter. Subesequent research should focus on the shape factor value for gradings of significantly different coarseness and gradings that consist of a significantly higher percentage of relatively elongated or blocky rock compared with the tested gradings, the shape factor will increase. For uncrushed rock it is expected both interests are met.