The density of the liquid phase of normal alkanes at the normal boiling point is represented by an empirical equation based on two known variables : the temperature of the normal boiling point and the ratio of the mole weight to the sum of the atomic numbers of all the atoms in the molecule (the "alkane number"). This equation is also valid, when applied to boiling points of different but constant pressures, but with specific coefficients for each pressure. The correlation between these coefficients and vapor pressure leads to one equation for the density of the liquid phase of the first 17 n-alkanes at vapor pressures from 1.333 to 101.325 kPa. Molecular weight and alkane number are also shown to give accurate correlations of the critical density, the liquid density at constant temperature and pressure, and the refractive index at constant temperature and pressure.