Abstract Prediction of disc cutter wear extent is a critical factor for improving the working performance of tunnel boring machines (TBMs). In this study, three TBM working performance parameters are demonstrated to be directly influenced by disc cutter wear and selected as factors that predict disc cutter wear extent; new equations that describe the specific energy (SE) requirement of all disc cutters on the cutterhead are developed, and the SE rule is revealed based on mechanical analysis of the disc cutters as they cut hard rock. Two particular project cases are analysed to verify the above research results. The SE rule varies with disc cutter radius, while the change in disc cutter radius directly reflects the cutter wear extent. Consequently, the cutter wear extent can be predicted utilising the SE rule. Furthermore, the wear mechanism is investigated for the prediction of disc cutter wear extent. Lastly, a novel energy method is established that is based on the SE rule and the wear mechanism. In application, the average wear extent of all disc cutters on the cutterhead can be calculated using the SE equations, and the wear extent of each cutter can be predicted according to wear mechanism and the layout of the disc cutters on the cutterhead. To illustrate the applicability of this energy method, data from on-site Qinling tunnel boring are analysed in detail as an example. The establishment of this method is based on mechanical analysis of the disc cutter as it cuts hard rock; the parameters used in this method reflect rock mass properties and vary from moment to moment as disc cutter wear changes. Therefore, this method is well founded and can be used for real-time prediction of disc cutter wear for hard rock TBMs.