A quantitative method, based solely on optical absorption, to determine the total iron (Fe) concentration in Fe : LiNbO₃ is proposed. Absorption spectra of several samples doped by thermal diffusion with different concentrations and different [Fe²⁺]/[Fe³⁺] ratios show an isosbestic point at 342 nm. At this wavelength the absorption is proportional to the total Fe concentration and does not depend on the oxidation state. Thanks to the large number of samples covering a wide range of concentrations, in this work it was possible to estimate an effective absorption cross-section relating the absorbance of a given sample to its iron content. The main advantage of the proposed method is in its simplicity and the fact that the result does not depend on the reduction degree of the sample. As it is known that the absorbance of Fe : LN at another wavelength (532 nm) gives information on the amount of Fe²⁺ present in the sample, our method makes it possible to characterize both the total Fe amount and its reduction degree within a single optical absorption measurement.