A rigorous theoretical model is derived and used to calculate reverberation time of a room in terms of impedance of absorption materials on the wall and geometrical factors associated with such materials and the room. In addition, the reverberation time also depends upon the initial state of the pressure field before decay begins as well as on the single measure of pressure, for example spatial root mean square, used to characterize the decay-time history. The theoretical model, which consists of a system of harmonic oscillators with damping provided through the wall impedance, is sufficiently general to take into acount all of these factors. Numerical examples are given to illustrate the procedure. A brief review of other methods for calculation of reverberation time is given, including a discussion of random and normal absorption coefficients.