This method is for calculating the UOP Characterization Factor of petroleum oils from API gravity and distillation or viscosity data. Average molecular weight of petroleum oils is estimated from API gravity and distillation data. The UOP Characterization Factor, commonly called K, is indicative of the general origin and nature of a petroleum stock. Values of 12.5 or higher indicate a material predominantly paraffinic in nature. Highly aromatic materials have characterization factors of 10.0 or less.