The "line ratio method" (Stenflo, 1973) has been extensively used in the past to carry out measurements of the magnetic field strength in spatially unresolved magnetic flux concentrations. The authors present here a new variant of this technique, which is particularly simple as it does not depend on any radiative transfer calculations and thus the assumption of a model atmosphere is not required. General properties of the transfer equation lead to a relationship between the circular polarization generated by two lines which are identical except for their Landé factors. This can be used to directly determine the field strength from the measured line profiles. In order to test the method the authors have applied it to experimental data. A comparison with the traditional line ratio method is shown.