Indoor carbon dioxide (CO 2 ) concentrations have been used for decades to characterize building ventilation and indoor air quality. Many of these applications require rates of CO 2 generation from the building occupants, which are currently based on approaches and data that are several decades old. However, CO 2 generation rates can be derived from well-established concepts within the fields of human metabolism and exercise physiology, which relate these rates to body size and composition, diet, and level of physical activity. This paper reviews how CO 2 generation rates have been estimated in the past and discusses how they can be characterized more accurately. Based on this information, a new approach to estimating CO 2 generation rates is presented, which is based on the described concepts from the fields of human metabolism and exercise physiology. Using this approach and more recent data on body mass and physical activity, values of CO 2 generation rates from building occupants are presented along with the variability that may occur based on body mass and activity data.