The approach is based on considering the excluded area, that is similar to excluded volume in 3D systems. The novelty concerns the repulsive component of the equation of state accounting the particles' eigen sizes and doesn't touch the attraction forces, for which the standard calculation methods are implied. The approach is equally applicable to monolayers composed of neutral or charged particles of any nature and size. In particular, it's applicable to 2D systems of nanoparticles transiting from free motion to nanostructure formation.