Seven empirical correlations for the calculation of the normal boiling density of non polar pure fluids are studied for their accuracy and applicability. One of the correlations is specific only for the normal boiling density, five are based on the corresponding states method (these need the critical parameters and acentric factor as inputs), and the other is a new kind of correlation that uses the Lennard-Jones molecular parameters and the acentric factor as input data. As reference, we take the value for the normal boiling density given by the specific correlation functions proposed in the DIPPR project for 252 nonpolar fluids, grouped into 17 families. In view of the results, we recommend the use of the Yamada-Gunn expression because of its simplicity and because other, less simple, models represent no clear improvement over its accuracy. We also recommend the use of the molecular-parameter model because it gives very good results for some families of fluids.