This report is a part of a thesis work at the division of Fluid and Mechatronic Systems(Flumes) at Linkoping University. The aim of this thesis is to build a robust, advanced, simple, easy to maintain, easy to develop and user-friendly program for the general aviation aircraft conceptual design. The program name is FLEX (FlumesExcel). The program was developed based on the most famous references in this eld such as Raymer [1], Gudmundsson [2] and Torenbeek [3]. Dierent methods and equations were evaluated to choose the best. In case there is no way to evaluate the methods, they were all implemented and the user has the ability to choose the desired one. Microsoft Excel was chosen to build the program and Excel VBA was used to build macros and functions in order to serve the objectives of the program. This report explains the used methods, the implementation way, and the program arrangement. It also shows how the results are presented in the program and provides the user with notes about the program using and its limits. The author supposes that the reader is familiar with the basic aerodynamic and aircraft design knowledge and nomenclatures. In order to not extend the report, some methods and theories are referred to the references without deep explanation. In case the used equations were derived by the author, they are explained in detail.