In order to develop cost-efficient software it is crucial to measure the accurate software size. However; measuring the software size has up to now been almost entirely a manual process and, as such, is both time-consuming and prone to human error which can end up with time and money loss. Automation of this process is a must for the software developing companies to improve the quality of project and budget planning. This paper introduces a mapping between COSMIC concept elements and UML conceptual elements and an automation tool in order to capture the information needed for functional software size measurement from UML diagrams in a component based software product line environment (CBPL) that are structured in accordance with the interface based design (IbD) method. The mapping and the tool combined is called PL FSM. The results obtained by manual measurement and automated measurement are compared and the results are observed to be close. As a result of this study, PL FSM approach is validated in CBPL environment at the design stage of software development process. The case studies have been carried out in embedded systems domain.