Service Oriented Architectures (SOA) mainly rely on the service composition mechanism which supports the reusability. Numerous researches focus on this thematic in order to satisfy all the objectives of quality fixed by the SOA community. One of these objectives is the notion of loose coupling between the composition's services. However this notion is intuitively understood and tangible, its definition lacks formalism. This unclear picture of the loose coupling is reflected in limited metrics which do not allow for its quantitative and objective evaluation. In this paper, we propose a set of different metrics which are associated with a clear definition of the loose coupling notion. These metrics act as guidelines for architects to develop their composite service. They are gathered in a formula which allows for evaluating the global coupling of a composition of services. Moreover, they are also used as comparison criteria for classifying existing composition models.