Non zero signal rise and fall times significantly contribute to the gate propagation delay. Designers must accurately consider them when defining timing library format. Based on a design oriented macro-model of the timing performance of CMOS structures, we present in this paper a general representation of transition times allowing fast and accurate cell performance evaluation. This general representation is then exploited to define a robust characterization protocol of the output transition time of standard cells. Both the representation and the protocol are finally validated comparing calculated gate input-output transition time values with standard look-up representation obtained from Hspice simulations (Bsim3v.3, level 69, 0.25Μm process).