For a long time, scientific research has tried to establish the relationships between jet momentum and room velocities. The final breakthrough is still to come. One approach is to use a kinetic energy balance, which was initially suggested by Elterman (1980). This paper presents a thorough kinetic energy analysis. Based on the analysis, a new method is developed for calculating the average room velocity. The calculation method is evaluated with the experimental date from laboratory experiments with three different air distribution methods. An excellent correlation is found between the kinetic energy method and the experimental date within the occupied zone for two air distribution methods and two rooms scales. With the third air distribution method, vertical air supply, a linear correlation is found, but the calculated values are smaller than the measured velocities because the occupied zone is partly within the main jet area.