It is widely accepted that the aerodynamic diameter of a particle is one of the main factors which determine particle deposition into the human respiratory system. The determination of aerodynamic diameter of spheres or near spherical objects is routinely accomplished using impactors. The aerodynamic diameter of fibers, on the other hand, have not traditionally been measured by impactors, because fibers of the same cross section can have different lengths and a variety of shapes (straight, curve, etc.) for each length. Thus, the aerodynamic size of the fibers is a function of fiber orientation. Nevertheless, fibers do have an aerodynamic diameter, albeit a function of orientation, and will be collected in an impactor if the aerodynamic diameters are larger than the cut size of the impactor. Single and multiple stage impactors have been developed, calibrated and validated specifically for the determination of the aerodynamic diameter of large fibers with physical diameters of 20-50 µm and lengths between 75 and 1000 µm. The performance characteristics are evaluated by round particles of known diameters, fibers of known length and diameter, and computational fluid dynamic calculations. Our methodology has shown that inertial impactors can be used to experimentally determine the aerodynamic diameter of large cellulose acetate fibers.