CalculiX allows users to create, analyze, and process finite element models efficiently. It features an interactive 3D pre-and post-processor that utilizes the OpenGL API for enhanced visualization. The solver within CalculiX is capable of handling both linear and non-linear calculations, offering solutions for static, dynamic, and thermal problems. Since it employs the Abaqus input format, users can leverage commercial pre-processors seamlessly. Furthermore, the pre-processor can generate mesh-related data compatible with Nastran, Abaqus, Ansys, Code-Aster, as well as free computational fluid dynamics tools such as Dolfyn, Duns, ISAAC, and OpenFOAM. A straightforward step reader is also integrated into the system. Additionally, there are options for external CAD interfaces, broadening its usability. This versatile program is designed to operate on various Unix platforms like Linux and Irix, as well as on MS Windows, making it accessible to a wide range of users.