Gels are complex structures that can be described in terms of the fractal dimension, d(f), of the clusters that constitute them. Classical techniques, based on the structure factor obtained through scattering measurements, provide erroneous values of d(f), which differ from the values estimated from the corresponding particle-density correlation function. The source of this error is identified in this work, and a procedure to get the correct value of df from experimental scattering data is indicated.