Much attention has been paid to study the state of charge (SOC) during fast filling process. However, investigation on identifying the foremost factors and contribution of them to the filling mass is still an open issue. In essence, the contributing factors are multiple, of which, the most important factors can be found out by thermodynamical analysis. Based on the thermodynamical analysis and mass flow rate balance, some equations calculating filling mass and heat transfer are outstanding. The mass filling rate, the initial pressure in the cylinder and the inlet temperature of hydrogen are confirmed to be the utmost important factors influencing the filling mass. A computational fluid dynamics (CFD) model is established. The simulation results show the liner or inverse proportional relationship between filling mass and the three factors, hence, a formula for the final mass with diverse filling conditions is figured out. By means of studying the state of charge obtained by adiabatic and diathermal filling processes, a formula to investigate the heat transfer is proposed. Both the filling mass and heat transfer are coupled with the three factors. Therefore, the effect of heat transfer on filling mass has been investigated. It seems that the filling mass is dependent on the total heat transfer during fast filling. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.