This paper introduces the open-source GridTool software available at GitHub. It is an easy-to-use tool to convert and simplify electricity grid data from the OpenStreetMap-project for the use in energy simulation models. The main advantages of the newly developed tool are its flexibility, customizability and its heuristic approach for identifying grid nodes. Instead of the lengthy and predominantly manual process of converting and implementing electricity grid data from transmission and distribution system operators, the GridTool shortens that task to seconds. During this process the OpenStreetMap data gets reduced to the essential things that are needed in energy simulation models for load flow purposes, with optional plots that help to tweak the most important settings. To our knowledge there is no other open-source tool, that works out-of-the-box that does this task and is as easy-to-use, only needs one program to run (MATLAB) and uses a heuristic approach.