Customarily structure factor S(Q) of a liquid is determined experimentally (by X-ray or neutron scattering methods) and is used to derive the radial distribution function g(r) which obviously is a laborious and costly procedure. In the present paper, an equation for S(Q) for rear fluids is derived which incorporates directly the internal pressure, pi (i), based on the fact that, the radial distribution function and hence the structure factor of a hard-sphere and a real fluid are similar. The positions of the principal maxima (PPM) correlate nicely with those adduced by experimental techniques for condensed liquids and liquid metals. In the (X-ray or neutron) scattering experiment, Q refers to the momentum change of the incident beam. In our study Q continues to have the same meaning in the 'hypothetical' experiment.