A controller for a common-rail injection system includes a plurality of fuel injectors, a common fuel supply line for the fuel injectors, a high-pressure pump for supplying the common fuel supply line with fuel, and a pressure sensor for determining the pressure in the common fuel supply line. A determination unit evaluates data of the pressure sensor and, from the pressure drop caused by an injection in the common fuel supply line, determines the fuel quantity actually injected during this injection or a value derived therefrom. An adaption unit uses the results of the determination unit in order to adapt the actuation of the fuel injectors. The determination unit carries out at least one test injection, and the actually injected fuel quantity or a value derived therefrom is effected by way of the test injection or injections.