Indoor carbon dioxide (CO2) concentrations have been used for decades to purportedly evaluate indoor air quality (IAQ) and ventilation. However, many applications of CO2 as a metric have reflected a lack of understanding of the connection between indoor CO2 levels, ventilation and IAQ. In many cases, an indoor concentration of 1800 mg/m3 (1000 ppmv) has been used as a metric of IAQ and ventilation without understanding its basis or significance. After many years of effort trying to dissuade practitioners as well as researchers from using this value, or some other concentration, as a metric of ventilation and IAQ, the author has developed an approach to determine a CO2 level that can be used as a meaningful indicator of the outdoor ventilation rate per person. Rather than a single CO2 concentration for all spaces and circumstances, this paper describes an approach to estimating a space-specific CO2 concentration from several relevant factors. The concept is based on an estimate of the CO2 concentration that would be expected in a specific space type given its intended or expected ventilation rate per person, the number of occupants and the rate at which they generate CO2, and the occupancy schedule. A calculation method is described for estimating the CO2 concentration for a given space and the timeframe for achieving that concentration, which provides a more meaningful metric than a single value for all spaces.