Abstract The frequency, intensity and duration of heat waves are all expected to increase as the climate warms in response to increasing greenhouse gas concentrations. The focus of this study is on another dimension of heat waves, their spatial extent, something that has not been studied systematically by researchers but has important implications for associated impacts. Of particular interest are spatially contiguous heat wave regions, examined here over the conterminous US for the May–September season in both the current climate and climate model projections from the CMIP5 archive (11 models total) using the RCP4.5 and RCP8.5 radiative forcing scenarios. Given their myriad impacts, heat waves are defined using multiple temperature variables, one which includes atmospheric moisture. In addition to their spatial extent, several other physical attributes are computed across contiguous heat wave regions, including a proxy for energy use. An estimate of the human population exposed to current and future heat waves is also evaluated. We find that historical climate model simulations, in aggregate, show good fidelity in capturing key characteristics of heat waves in the current climate while projections show a substantial increase in spatial extent and other attributes by mid-century under both scenarios, though generally less for RCP4.5, as expected. Overall, the study presents a framework for examining the behavior, and associated impacts, of a frequently overlooked aspect of heat waves. The projected increases in the spatial extent and other attributes of heat waves reported here provides a new perspective on some of the potential consequences of the continued increase in atmospheric greenhouse gas concentrations.