A method for recalibrating the injection law of a fuel injector to be tested in an injection system comprising: a plurality of fuel injectors, a common rail feeding the fuel under pressure to the fuel injectors, and a fuel pump which keeps the fuel under pressure inside the common rail; the method comprises the steps of: establishing, during a step of design, a set of characteristic actuation times (t1, t2, t3, t4) which allow to reconstruct with adequate accuracy the injection law of the fuel injector to be tested; determining the desired fuel quantity (Qd) for the fuel injector to be tested as a function of the objectives of the engine control unit of an internal combustion engine using the injection system; completely interrupting the feeding of fuel from the fuel pump to the common rail; avoiding the opening of all the other fuel injectors except for the fuel injector to be tested; measuring the initial fuel pressure (Pi) inside the common rail before starting the opening of the fuel injector to be tested; choosing a test actuation time which is compatible with the desired fuel quantity (Qd) from the predetermined set of characteristic actuation times (t1, t2, t3, t4); performing at least one first measurement opening of the fuel injector to be tested with a test actuation time to inject as a whole a first amount of fuel lower than the required fuel quantity (Qd); measuring the final fuel pressure (Pf) inside the common rail after having ended the first measurement opening of the fuel injector to be tested; determining a pressure drop (ΔP) in the common rail during the first measurement opening of the fuel injector to be tested, which is equal to the difference between the initial fuel pressure (Pi) and the final fuel pressure (Pf); and estimating, as a function of the pressure drop (ΔP) in the common rail, the fuel quantity which is actually injected by the fuel injector to be tested when it is opened for the test actuation time; the method is characterized in that it comprises the further steps of: determining a first fuel quantity, which is fed in total during the first measurement opening; calculating a second fuel quantity as the difference between the desired fuel quantity (Qd) and the first fuel quantity; determining a completion actuation time as a function of the second fuel quantity and using the current injection law; and performing, immediately after the first measurement opening, a second single completing opening of the fuel injector to be tested with the completion actuation time, so as to feed the second fuel quantity, which is necessary to reach the desired fuel quantity (Qd). establishing, during a step of design, a set of characteristic actuation times (t1, t2, t3, t4) which allow to reconstruct with adequate accuracy the injection law of the fuel injector to be tested; determining the desired fuel quantity (Qd) for the fuel injector to be tested as a function of the objectives of the engine control unit of an internal combustion engine using the injection system; completely interrupting the feeding of fuel from the fuel pump to the common rail; avoiding the opening of all the other fuel injectors except for the fuel injector to be tested; measuring the initial fuel pressure (Pi) inside the common rail before starting the opening of the fuel injector to be tested; choosing a test actuation time which is compatible with the desired fuel quantity (Qd) from the predetermined set of characteristic actuation times (t1, t2, t3, t4); performing at least one first measurement opening of the fuel injector to be tested with a test actuation time to inject as a whole a first amount of fuel lower than the required fuel quantity (Qd); measuring the final fuel pressure (Pf) inside the common rail after having ended the first measurement opening of the fuel injector to be tested; determining a pressure drop (ΔP) in the common rail during the first measurement opening of the fuel injector to be tested, which is equal to the difference between the initial fuel pressure (Pi) and the final fuel pressure (Pf); and estimating, as a function of the pressure drop (ΔP) in the common rail, the fuel quantity which is actually injected by the fuel injector to be tested when it is opened for the test actuation time; the method is characterized in that it comprises the further steps of: determining a first fuel quantity, which is fed in total during the first measurement opening; calculating a second fuel quantity as the difference between the desired fuel quantity (Qd) and the first fuel quantity; determining a completion actuation time as a function of the second fuel quantity and using the current injection law; and performing, immediately after the first measurement opening, a second single completing opening of the fuel injector to be tested with the completion actuation time, so as to feed the second fuel quantity, which is necessary to reach the desired fuel quantity (Qd). determining a first fuel quantity, which is fed in total during the first measurement opening; calculating a second fuel quantity as the difference between the desired fuel quantity (Qd) and the first fuel quantity; determining a completion actuation time as a function of the second fuel quantity and using the current injection law; and performing, immediately after the first measurement opening, a second single completing opening of the fuel injector to be tested with the completion actuation time, so as to feed the second fuel quantity, which is necessary to reach the desired fuel quantity (Qd).