A theoretical technique for determining equivalent aerodynamic diameters (EAD) of irregularly shaped particles was developed and verified experimentally. High speed computers were used to solve two dimensional and three dimensional Navier Stokes equations on spheres, cylinders, disks and cubes. In all cases, calculated values agreed within 5% of reported experimental values. Three dimensional algorithms were then used to determine flow fields around irregularly shaped particles. These values were used to determine drag force, from which EAD could be calculated. Experimental verification was done using coal, silica and talc particles, 1 to 4 microns in diameter, analyzed using a spiral duct centrifuge and scanning electron microscopy (SEM) with two dimensional orthogonal shadowing. The centrifuge separated irregular particles based on size and orientation. In SEM, particles were shadowed with gold film in two orthogonal directions at a 15 degree angle. This provided shape and size parameters in three dimensions. The numerical calculation defined particles as groups of blocks around which flow fields were calculated. From these, drag force and EAD values were determined. Theoretical and experimental values revealed good agreement. Particle orientation contributed variations of as much as 17% in measured diameters, and this could be accurately calculated by the numerical technique. The authors conclude that the theoretical technique is a powerful tool for determining EADs of irregularly shaped particles in any orientation.