It also depends on the morphology of the particles. The formula varies between rods, sphere, cubes and likes. You can characterize SSA using Brunauer–Emmett–Teller.
Ali Nematollahzadeh The initial calculation of SSA is actually m2/m3. With a knowledge of the density this can be converted to m2/kg. The equation above reflects unit density and thus D[3,2], the Sauter Mean Diameter, is in units of m3/m2 or meters.