The mass of a particle is computed by summing the masses of all bulk species molecules in the core. According to the particle class definition, the mass of a class j particle becomes

(6–1)

The mass of a single bulk species molecule is assumed to be constant and can be calculated from

(6–2)

where is the molar weight of the bulk species and is the Avogadro number. The volume of a class j particle can be calculated from its mass and bulk density of the particle core as

(6–3)

The bulk density of the particle core has implicitly accounted for the effect of molecule packing inside the core and its value is given when the bulk species is declared. It thus can be seen that both particle mass and volume are proportional to its class. Up to this point, there is no assumption regarding the shape of the particles so Equation 6–1 to Equation 6–3 are generally applicable to particles of all shapes.