A simple and better method for the prediction of the molecular weight of petroleum fractions (C5-C120) is presented. It requires as input information the 50 wt % true boiling point (TBP) as derived from simulated distillation and the density only. It is demonstrated that the 50 wt % TBP of fractions is equivalent to the normal boiling point of pure compounds. This finding has facilitated the development of the pertaining data set, which is included. Excellent results are obtained with a standard deviation of about 2%, which is at least 3 times better than that of published methods, whereas direct measurements of molecular weights are time consuming and often less accurate. The new method is compared to the API procedure 2B2.1 (1987), which is the current industry standard but limited to molecular weights of 700, and encompasses now the full practical range of molecular weights of 75-1700.