Abstract: A system and a method of measuring a particle's size in a select aerosol using the optical diameter of the particle to perform a mobility and/or aerodynamic diameter conversion without any knowledge about the particle's shape and its optical properties in the aerosol being characterized. In one example embodiment of the invention, the method includes generating a set of calibration data and finding the optimal refractive index and shape that best fits the calibration data. In addition, the method includes creating a new calibration curve that provides a mobility-equivalent or aerodynamic-equivalent diameter.