A ‘size-strength’ graph, subdivided for various methods of excavation, was published by Franklin, Broch & Walton in 1971. Although this graph allows excavatability to be assessed rapidly, the subdivisions have become outdated as more powerful, more efficient equipment has become available. A complete revision of the graphical method, based on more than 100 case studies, is proposed. Particular emphasis is placed on rock masses which can be broken up solely by mechanical means such as ripping. Procedures are recommended for obtaining geotechnical data, and the importance of interpreting field observations in their geomorphological context is discussed. Consideration is also given to the effects of block shape and orientation on ripper performance, and adjustments to the input data are suggested. The revised graph is intended for all types of project but especially for road construction, where plant mobility and flexibility are important.