The discovery of graphene by Andre Geim and Konstantin Novoselov in 2004 contributed significantly to complementary metal-oxide-semiconductor (CMOS) technology, especially for electronic devices. Due to its outstanding properties, graphene has been used in many applications such as field-effect transistors (FETs), touch-sensitive screens, liquid-crystal displays, light-emitting diodes, dye-sensitised solar cells, and organic solar cells. Consequently, interface charge density in graphene field-effect transistors has become an important area of research in order to more accurately determine electronic characteristics such as capacitance-voltage (C-V). In this research, we developed a comprehensive simulation tool based on graphene FET called graphene field-effect transistor simulator (GFETSIM) of interface charge density, which functions as a graphical user interface (GUI) using MATLAB. This simulation represents the on-going development of a carbon-based device simulator begun in 2014 at Universiti Teknologi Malaysia (UTM). An automated selfconsistent approach is utilised to extract and compute the interface charge density. It is coded in MATLAB and the program was developed into an interactive MATLAB GUI simulation tool. The interface for GFETSIM is generated using the graphical user interface development environment (GUIDE) in MATLAB. This simulator allows users to import external C-V data into the GUI and carry out analysis based on the density of states plots. In addition, key parameter values can be varied to assess the outcome and changes in interface trapped density. In addition, a reset button is embedded into the simulator.