Although volcano instability is increasingly recognized as a societal hazard, numerical data on the relevant mechanical properties remain sparse. We report new field data on the rock-mass properties of volcanic materials from Snowdonia (North Wales, UK) and Tenerife (Canary Islands). Using rock types that range in composition from phonolite to rhyolite, we summarize a method for estimating the overall strength of a volcanic edifice based on the rock-mass rating index (RMR) and the Hoek–Brown criterion. We show that the average rock-mass compressive (σcm) and cohesive (c) strengths decrease exponentially with RMR according to σcm=0.652(0.0559RMR) and c=0.035(0.0669RMR), respectively, and appear insensitive to both initial magma composition and relative age. This exponential relationship provides a new predictive tool for directly estimating rock-mass strength from the RMR. Our analysis further predicts a marked reduction of up to 96% in the rock-mass compressive strength relative to the intact rock value based on laboratory tests and that, overall, the combined results from both study areas yield cohesive strength values from 4.8 to 0.44 MPa. Estimated values of rock-mass angle of friction range from 28° to c. 38°. Recent modifications to the Hoek–Brown criterion, in particular the inclusion of the disturbance factor D, suggest that even these low values of rock-mass strength and cohesion may be optimistic, and the true values of rock parameters relevant for accurate predictions of volcano edifice strength may be up to 30% weaker still.