RTMsim is a robust, easy-to-use and simple-to-extend software tool, implemented in Julia (Bezanson et al., 2017) for resin transfer moulding (RTM) filling simulations. A shell mesh, injection pressure, resin viscosity and the parameters describing the preform are required input. The software was validated with results from literature (Advani et al., 1994) and compared with results from well-established RTM filling simulation tools (Gion Barandun et al., 2012),(ANSYS, 2021).