An analytical framework for calculation of Erlang capacity per cell of UMTS with perceptual-based power control is derived. A comparison of the capacity of the system using the proposed power control algorithm with its conventional counterpart is performed. This is supported by numerical results on Erlang capacity over a variety of system parameters for both considered power control approaches. It is shown that the use of perceptual speech quality metrics in power control results in a capacity gain of at least 10% over that of conventional UMTS.