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The grading, whether in terms of mass or size distribution of rock, is critical in the design of hydraulic civil engineering works such as breakwaters, coastal protection revetments, river training works and dam face protection. In the quarry, more effort is needed to control these rock properties than all the others. However, in practice, a poor level of understanding of relatively simple statistical concepts and the recent introduction of an elaborate specification system have contributed to many pitfalls in quality control. The acceptable size and mass ranges for consignments of rock materials are set down in the European standard for armourstone, EN 13383:2002 ( BSI 2002 ). Armourstone pieces are substantially coarser than traditional aggregates and typically include large rock blocks sometimes weighing several tonnes. This standard, in force since 2004, introduced a system of standard gradings and requirements. Although similar to that promoted in CIRIA/CUR (1991) , a detailed explanation is long overdue. In this paper, the Rosin–Rammler equation for cumulative mass and size distributions is adopted. A purchaser or designer can then visualize from a single idealized curve, together with acceptance limits, the most reasonable expectation for a specified grading, rather than work with an envelope of acceptable grading limits. The research also draws upon actual mass distributions measured from projects dating from 1991. These are combined with theoretical data sets. The result is a new expression for what has become a contractually important ratio. The ratio is that between the mean mass as given by bulk weighing and counting, and the median mass used by designers. Non-standard grading specification is also treated, providing further background to recommended best practice as presented in the forthcoming update to the 1991 rock manual.