To use the RC approximations described in Sections 5.5 and 5.6, it is necessary to estimate the full-on resistance of a transistor. The full-on resistance is the resistance seen between drain and source of a transistor assuming the gate voltage is constant and the gate is fully conducting. This resistance can also be used for pass transistors that (as far as the critical path is concerned) are fully conducting.
It is assumed that the equivalent resistance of a conducting transistor is inversely proportional to the transistor width (only minimum-length transistors were used). Thus,
where R is a constant (different for NMOS and PMOS transistors) and W is the transistor width.