It is defined as: the surface area per powder mass, in m2/g or cm2/g. When the powder particles have no voids, the specific surface area SW = k/PD, P is the density of the powder, and K is the shape coefficient of the particles. For spherical particles k=6, different shapes have different coefficients. D is the average particle size of the powder. Therefore, as long as the average particle size of powder is known, its specific surface area can be calculated. Of course, the shape coefficient of non-spherical particles should be corrected. This is the principle that modern particle size analyzer can measure specific surface area data.