If there are two spheres of radii  and  on the  axis, and the spheres are  distance apart, where  is much smaller than  and , then the force, , in the  direction is
In this equation, , and  is the normal force per unit area between two flat surfaces distance  apart. 
When the Derjaguin approximation is applied to depletion forces, and 0<h<2Rs, then the depletion force given by the Derjaguin approximation is
In this equation,  is the geometrical factor, which is set to 1, and , the interfacial tension at the wall-fluid interface.