An extended Hermans-Ende equation for real solutions has been derived in order to calculate the density as a function of the radial distance in a density gradient mixture (light/heavy medium) inside an ultracentrifugal cell. The equation has been tested with equilibrium density gradient measurements of the system water/metrizamide/polystyrene latex where metrizamide is the heavy medium. The solvent compositions were 8 to 15 mass-% metrizamide (2-(3-acetamido-5-N-methylacetilmido-2,4,6-triiodobenzamido)-2-deoxy-D-g lucose), The latex concentration was 0.03 mass-%. It is demonstrated that the density gradient is highly influenced due to nonideality effects with respect to the solvent composition and due to the pressure gradient in the ultracentrifuge.