A method for operating a fuel injection system of an internal combustion engine is described. The fuel injection system has an injector for metering fuel into a combustion chamber of the internal combustion engine. The injector has an actuator, a control valve and a nozzle needle. In the method, the actuator is supplied a voltage and/or a current during an activation period. The control valve is set into a lifting motion by the actuator. The injector is opened and closed by the nozzle needle, using the lifting motion of the control valve. A further time period is ascertained. The further time period ends at the closing time of the nozzle needle. A function is ascertained, which links the activation period to the further time period. A shortest activation period, for which the nozzle needle opens and produces an injection, is ascertained with the aid of the function. An opening delay period (do1) of the nozzle needle is ascertained as a function of the shortest time period.