Pharmaceutical aerosols are used to treat many pulmonary diseases. The use of low-density powders has proven useful to support efficient drug delivery. Measurements must account for the low-density, spherical particle features contributing to aerodynamic behavior. Ideally, the aerodynamic particle size distribution (APSD) is measured experimentally. Without formal measurement of APSD, calculations may be performed using surrogate measures such as bulk or tapped density and dynamic shape factor in Stokes' equation.