The concepts leading to the production of a FORTRAN computer program to predict the output power of a C0 2 laser to be constructed are presented. In our calculation optical and geometrical parameters are taken into account. The physical assumptions and approxirnations made the program restricted for applications to lasers producing from 5 to 250 watts working in continuous wave mode the computer prediction and the experimental results certainly differ, but they are a rough guide to the proper functioning of a new design, such is the case when the construction conditions of a laser developed at the Physics Departament were entered in the program.