A method for determining 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 injectors, and a fuel pump which keeps the fuel under pressure inside the common rail; the method comprising the steps of: establishing, in a design phase, a test actuation time; completely interrupting the feeding of fuel from the fuel pump to the common rail; avoiding the opening of all the fuel injectors except for the fuel injector to be tested; measuring the fuel initial pressure (Pi) inside the common rail before starting the opening of the fuel injector to be tested; opening the fuel injector to be tested for a number of consecutive openings greater than one with a same test actuation time; measuring the fuel final pressure (Pf) inside the common rail after ending the opening of the fuel injector to be tested; determining a pressure drop (ΔP) in the common rail during the opening of the fuel injector to be tested, which is equal to the difference between the fuel initial pressure (Pi) and the fuel final pressure (Pf); estimating, according to 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; carrying out, during the normal use of the internal combustion engine, a series of estimates of the fuel quantity which is actually injected by the fuel injector to be tested when it is opened for a same test actuation time; and statistically processing the series of estimates of the fuel quantity, in order to determine an average 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 the desired fuel quantity (Qd) for the fuel injector to be tested according to the objectives of the engine control unit of an internal combustion engine using the injection system; determining the desired actuation time (Td) for the fuel injector to be tested according to the desired fuel quantity (Qd); comparing the test actuation time with the desired actuation time (Td), in order to establish whether the test actuation time is compatible with the desired actuation time (Td); and carrying out an estimate of the fuel quantity which is actually injected by the fuel injector when it is opened for the test actuation time, only if the test actuation time is compatible with the desired actuation time (Td). establishing, in a design phase, a test actuation time; completely interrupting the feeding of fuel from the fuel pump to the common rail; avoiding the opening of all the fuel injectors except for the fuel injector to be tested; measuring the fuel initial pressure (Pi) inside the common rail before starting the opening of the fuel injector to be tested; opening the fuel injector to be tested for a number of consecutive openings greater than one with a same test actuation time; measuring the fuel final pressure (Pf) inside the common rail after ending the opening of the fuel injector to be tested; determining a pressure drop (ΔP) in the common rail during the opening of the fuel injector to be tested, which is equal to the difference between the fuel initial pressure (Pi) and the fuel final pressure (Pf); estimating, according to 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; carrying out, during the normal use of the internal combustion engine, a series of estimates of the fuel quantity which is actually injected by the fuel injector to be tested when it is opened for a same test actuation time; and statistically processing the series of estimates of the fuel quantity, in order to determine an average 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 the desired fuel quantity (Qd) for the fuel injector to be tested according to the objectives of the engine control unit of an internal combustion engine using the injection system; determining the desired actuation time (Td) for the fuel injector to be tested according to the desired fuel quantity (Qd); comparing the test actuation time with the desired actuation time (Td), in order to establish whether the test actuation time is compatible with the desired actuation time (Td); and carrying out an estimate of the fuel quantity which is actually injected by the fuel injector when it is opened for the test actuation time, only if the test actuation time is compatible with the desired actuation time (Td). determining the desired fuel quantity (Qd) for the fuel injector to be tested according to the objectives of the engine control unit of an internal combustion engine using the injection system; determining the desired actuation time (Td) for the fuel injector to be tested according to the desired fuel quantity (Qd); comparing the test actuation time with the desired actuation time (Td), in order to establish whether the test actuation time is compatible with the desired actuation time (Td); and carrying out an estimate of the fuel quantity which is actually injected by the fuel injector when it is opened for the test actuation time, only if the test actuation time is compatible with the desired actuation time (Td).