ORC systems are the most used system for heat recovery. The considerable impetus they have had in recent years has led to new configurations and the choice of different types of plant layouts and components. And one of the key components is the expander, and consequently all the study that leads to the definition of such a machine. The topic of this paper is the implementation of a calculation code for choosing the expander or comparing it with others,, in the case of small rated plants (lower than 30 kW). The built code allows the preliminary design of the expanders, providing all characteristic dimensions. At the moment, the calculation procedures for radial input turbines and screw expanders have been implemented, also providing a possible guideline for comparison, based on the various research carried out over the last few years by the accademic researchers and using all commercially available data too. The next steps are to add all design procedures for the various expanders (scroll, vaned, pistons, tesla) and to realize, based on the various tests and data, also an automotic evaluation criterion that shows the optimal choice and possible competitors