Stack effect is a major driving force of smoke movement in buildings. This paper presents a general method for evaluation of the location of the neutral plane for a space connected to its surroundings by any number of openings. A computer program, STACK, for analysis of the location of the neutral plane and resulting flows is presented along with example analyses. The examples show that the location of the neutral plane between a space and its surroundings is a weak function of temperature and a strong function of the size of openings. Further, the mass flow rate leaving a space due to stack effect is a strong function of temperature.