A system and a method for the automated pipetting of fluids are disclosed as well as a method for calibrating the system. A channel pump having at least one tip-sided port connected to a pipetting tip by a tip-sided pump conduit for generating a positive or negative pressure in the pipetting tip, and at least one reservoir-sided port connected to a system fluid reservoir by a reservoir-sided pump conduit is provided. The channel pump when operated at a nominal flow rate exhibits a fluid backflow caused by a pressure difference in the tip- and reservoir-sided pump conduits which results in an effective flow rate compared to the nominal flow rate. Such a pressure difference can be used to determine an extended pipetting period which extends a pipetting period of the channel pump operating at the nominal flow rate, and to calibrate the system.