In very deep submicron technologies, the parasitic capacitor and resistance can have a significant impact on propagation delays. The Elmore delay metric is widely used due to its efficiency and ease of use. However, it is well know that this method can have significant error on large RC-circuit. In this paper, we present a new method for determining the analytic waveform of the RC-circuit outputs. The accuracy of this method is demonstrated on several large RC interconnect circuits.