Air age is an important index to evaluate indoor air quality in ventilated rooms. The traditional definition of air age is limited to the indoor part (i.e., the air age at the inlets of air supply diffusers is assumed to be zero). Total air age extends the traditional air age concept by considering the influence of air delivery process on air age distribution. In this paper, we report the development of an algorithm to calculate total air age in a room ventilated by multiple air-handling units (AHUs). Compared to the conventional iteration method, the new algorithm is much faster in obtaining a solution of total air age. The total air age distribution in a large space ventilated by 16 AHUs is calculated using the algorithm.