Reducing sitting time in favor of increased physical activity is now a key recommendation for adults in the U.S. While the many health risks associated with sedentary time are understood, it is less clear how much time U.S. adults spend sedentary each day and where and how sedentary time is accumulated. This study was designed to describe the amount and type of sedentary behaviors U.S. adults engage in on an average day, using estimates from a freely available internet-based previous-day recall, Activities Completed over Time in 24 Hours (ACT24). Adults ages 20 to 75 (N=2,640) reported spending an average of 9.5 hours sedentary each day, and most of this time was accumulated at work and during leisure-time. Sitting during leisure-time accounted for 47% of all sedentary time reported, and virtually all of their discretionary time (82%) was spent sitting and watching screens (e.g., television, phones, computers). The results provide new insight into the amount and type of sedentary behaviors that U.S. adults engage in on a typical day and point to key behavioral targets for future etiologic and intervention research.