Basic equations are derived to relate the chromatographic separation time to operational parameters, which are chosen to ensure that they can be experimentally measured. The most important of these parameters are the available pressure-drop across the column, the extracolumrrcontributions to band broadening, the plate height of the column itself, the parameters that determine the plate height, the column length, and the separation factor. The derivation is presented in such a way as to stress the assumptions often implicitly introduced in the theory.