This report proposes the stage volume ratio (Vo/V) as an acoustic design factor for concert halls and presents an investigation of the dependence of the acoustic parameters of an auditorium on the stage volume through computer simulation. Firstly, the ratio of the stage volume to the total volume of a concert hall was defined as Vo/V through case studies of existing concert halls. It was verified using a simple model that the stage acoustic parameter STEarly and audience acoustic parameters G and C80 decreased, whereas, the reverberation time and early decay time increased with increasing Vo/V. Secondly, a computer simulation was performed for nine existing concert halls, while changing Vo/V from −20% to 30%. The room acoustic parameters exhibited the same patterns as suggested by the simple model. Vo/V significantly affected the bass ratio and bass index. A comparison of the effects of Vo/V and the sound absorption coefficient in nine concert halls revealed that Vo/V contributed approximately 15% to the reverberation and affected the bass characteristics more substantially. Thus, Vo/V is a critical design factor when determining the warmth of the audience acoustics. The study results could be used as a basis for acoustic design in the future.