Abstract. Due to the few computing resource planning options currently available in Grid computing, capacity planning, an old discipline for analyzing resource purchases, is simple to perform. However, once a commercial computing Grid is established, which provides many different resource types at variable prices, capacity planning will become more complex and the user will require support for handling this difficult process. The support could come from an online Grid Capacity Planning Service, which helps users with little IT expertise to make use of the Grid in a cost-effective manner. This Grid Capacity Planning Service is a stand-alone service, enabling companies to outsource their capacity planning task. This paper describes the Grid Capacity Planning Service and demonstrates the workings of the service through simulations.