This paper describes a technique for automating the size estimation of software projects conducted using Model Driven Development methods. Specifically, an algorithm has been implemented for performing the function point count of a software project starting from its conceptual model. Differently from previous approaches, the proposed technique works on the same conceptual model that is used for producing the implementation, eliminating any unnecessary ad-hoc specification task. We evaluate the precision of the FP computation algorithm on a set of real-world projects and describe its implementation within a commercial Model Driven Development tool suite.