Rock Quality Designation (RQD) provides a quantitative assessment of fracturing that is relevant to the mechanical behaviour of the rock mass from which it is measured. Its values are commonly incorporated into the geotechnical classifications that are used to assist both the characterisation of rock masses and the execution of preliminary designs for excavation and support within these rock masses. No measure exists of the certainty with which a value of RQD obtained in a particular direction adequately describes the range of RQD values which exist in that direction. This paper describes two new parameters, Rock Quality Percentage (RQP) and Rock Quality Risk (RQR), and explains how they reflect the extent to which a given value of RQD satisfactorily describes the amount of jointing in a given direction of measurement and hence the risks involved in allocating that value of RQD to the rock. Practical application of RQP and RQR has consistently exhibited their relationship to volumetric joint count (J_v) and suggests that these parameters have the potential to provide a quantitative prediction of the range of block sizes within a jointed rock mass, similar to the particle size distribution used so extensively in soil mechanics.