When both large colloidal particles and small depletants are in a suspension, there is a region which surrounds every large colloidal particle that is unavailable for the centers of the depletants to occupy. This steric restriction is due to the colloid-depletant hard-sphere potential.  The volume of the excluded region is
where  is the diameter of the large spheres and  is the diameter of the small spheres.
When the large spheres get close enough, the excluded volumes surrounding the spheres intersect. The overlapping volumes result in a reduced excluded volume, that is, an increase in the total free volume available to small spheres. The reduced excluded volume,  can be written
where  is half the width of the lens-shaped region of overlap volume formed by spherical caps. The volume available  for small spheres is the difference between the total volume of the system and the excluded volume. To determine the available volume for small spheres, there are two distinguishable cases: first, the separation of the large spheres is big enough so small spheres can penetrate in between them; second, the large spheres are close enough so that small spheres cannot penetrate between them. For each case, the available volume for small spheres is given by
In the latter case small spheres are depleted from the interparticle region between large spheres and a depletion force ensues.