Service Oriented Architecture (SOA) is rapidly emerging as the efficient approach in contemporary complex, heterogeneous computing environments. SOA increases the adaptability by loose coupling and its main feature is that three elements such as service provider, service requester and service registry are connected with each other systematically. To design the service-oriented system efficiently, a metric to measure the coupling between services accurately is needed. In this paper, we propose four coupling metrics for SOA based softwares. First, we suggest a coupling metric for service-oriented systems by modifying an established coupling metric of object-oriented systems. Then we suggest another coupling metric which includes indirect coupling between services. We also suggest two relative coupling metrics to measure coupling between subsystems. We investigate the theoretical soundness of the proposed metrics by the axioms of Briand et al. Finally, we apply the presented metrics to an industrial-scale case study.