A prototype commercial instrument and 2.1 mm i.d. columns packed with 1.7 microm porous particles have been used to measure peak capacity in ultra performance liquid chromatography (UPLC). Peak capacity was measured for a small molecule pharmaceutical as a function of gradient time, mobile phase flow rate, and column length. For very fast analysis, the highest peak capacity is obtained from a short column operating at high linear velocities. If an even higher peak capacity is required, a longer analysis time must be employed, and a point is reached where switching to a longer column becomes the best approach.