A method is developed to quantify thunderstorm intensity according to cloud-to-ground lightning flashes (hereafter ground flashes) determined by a lightning-location sensor network. The method is based on the ground flash density N_D per thunderstorm day (ground flashes per square kilometer per thunderstorm day) calculated on 20 km × 20 km fixed squares. Because the square size roughly corresponds to the area covered by a typical thunderstorm, the flash density for one square defines a unit thunderstorm for the purposes of this study. This method is tested with ground flash data obtained from two nationwide lightning-location systems: the National Lightning Detection Network (NLDN) in the contiguous United States and the portion of the Nordic Lightning Information System (NORDLIS) in Finland. The distribution of daily ground flash density N_D is computed for all of Finland and four 800 000 km² regions in the United States (identified as western, central, eastern, and Florida). Although Finland and all four U.S. regions have median values of N_D of 0.01–0.03 flashes per square kilometer per thunderstorm day—indicating that most thunderstorms produce relatively few ground flashes regardless of geographical region—the most intense 1% of the storms (as measured by the 99th percentiles of the N_D distributions within each region) show much larger differences among regions. For example, the most intense 1% of the N_D distributions is 1.3 flashes per square kilometer per thunderstorm day in the central U.S. region, but only 0.2 flashes per square kilometer per thunderstorm day in Finland. The spatial distribution of the most intense 1% of the N_D distributions illustrates that the most intense thunderstorm days occur in the central United States and upper Midwest, which differs from the maxima of the average annual flash density N_A and the number of thunderstorm days T_D, both of which occur in Florida and along the coast of the Gulf of Mexico. This method for using N_D to quantify thunderstorm intensity is applicable to any region as long as the detection efficiency of the lightning-location network is high enough or known. This method can also be employed in operational forecasting to provide a quantitative measure of the lightning intensity of thunderstorms relative to climatology.