The invention relates to a method for operating a fuel injection system of an internal combustion engine. The fuel injection system comprises an injector for the metered addition of a fuel to a combustion chamber of the internal combustion engine. The injector comprises an actuator, a control valve, and a nozzle needle. In the method, a voltage and/or a current is supplied to the actuator during an actuation period (dactive). The actuator causes the control valve to carry out a stroke motion. By means of the stroke motion of the control valve, the nozzle needle opens and closes the injector. A further period (dclose, dc1) is determined. The further period (dclose, dc1) ends with the closing time of the nozzle needle. A function is determined which links the actuation period (dactive) to the further period (dclose, dc1). By means of the function, a shortest actuation period is determined, at which the nozzle needle opens and an injection is carried out. An opening delay period (d01) of the nozzle needle is determined depending on the shortest duration (dactive,min).