The present invention provides a method for determining a particle size. The process includes the steps of: (i) positioning a particle on a flat surface; (ii) recording a digital image of the particle on the flat surface wherein the digital image is recorded with an image plane parallel to the flat surface and the digital image of the particle includes a digital background of the flat surface; (iii) removing the digital background image of the flat surface to obtain an isolated digital image of the particle; (iv) determining a surface area (A) of the particle from the isolated digital image of the particle; (v) tilting the particle at an angle (.theta.) with respect to the image plane to expose an edge of the particle; (vi) measuring a measured thickness (T.sub.m) of the particle; (vii) determining a calculated actual thickness (T) of the particle by the equation T=T.sub.m/sine .theta.; (viii) determining a particle volume (V) of the particle by the equation V=A*T; and (ix) determining an equivalent spherical particle size diameter (D) of the particle by the equation D=2*{square root over ((3/4)(1/.pi.)(V))}{square root over ((3/4)(1/.pi.)(V))}{square root over ((3/4)(1/.pi.)(V))}.