CalculiX is a package designed to solve field problems. The method used is the finite element method. With CalculiX Finite Element Models can be build, calculated and post-processed. The pre- and post-processor is an interactive 3D-tool using the openGL API. The solver is able to do linear and non-linear calculations. Static, dynamic and thermal solutions are available. Both programs can be used independently. Because the solver makes use of the abaqus input format it is possible to use commercial pre-processors as well. In turn the pre-processor is able to write mesh related data for nastran, abaqus, ansys, code-aster and for the free-cfd codes dolfyn, duns, ISAAC and OpenFOAM. A simple step reader is included. In addition external CAD interfaces like vda_to_fbd are available. The program is designed to run on Unix platforms like Linux and Irix computers but also on MS-Windows. The CalculiX package was developed by a team of enthusiasts in their raw spare time. They are employees of MTU Aero Engines in Munich, Germany which granted the publication.