A practical method is presented that can predict quantitatively the effect of extra-column band broadening on the observed plate number and resolution in high performance liquid chromatography (HPLC). This method is based on fundamental chromatographic equations that relate retention time, flow rate, maximum column efficiency, observed efficiency, column variance, observed variance, and extra-column variance. The parameter, Pl, is used to express the ratio of the system plate number actually observed for an analyte i in the presence of extra-column dispersion, relative to a (maximum) column plate number representing the absence of extra-column effects. The method is examined through 20 test cases encompassing four different analytes, a kl′i range of 0.7 to 19.7, a retention time range of 0.6 to 9.6 min, two different flow rates, two organic solvent modifiers, and four different detector and tubing configurations. Absolute values of Pl, determined experimentally agreed within 1 to 18% with values obtained by mathematical prediction of Pl.