One of the major current research trends in Software Engineering is the focus on the development of new techniques to deal efficiently with the design of systems that are able to evolve overtime and adapt to rapid changes of their requirements. However, it is still an open issue how to quantify and evaluate the adaptability of a given software system. In this paper we propose the definition of metrics able to quantify and evaluate such software adaptability at the architectural level. Besides, we define a relationship with the quality of service that the software must guarantee by means of a relation between the values of these metrics and the system quality requirements. The presented metrics can be used by the software architect to guide the system adaptation to fulfill the overall quality requirements.