The extent to which an irregularly shaped or non-spherical particle equates to being spherical. It is independent of particle size and defined as the ratio of the surface area of a particle to its actual surface area:
where dp is the equivalent or nominal particle diameter, vp is the volume of a particle, and sp is its surface area. For a spherical particle, Φ = 1. Tables of sphericity are used for particles of various defined geometric shapes.