This paper addresses the retrieval batch size which the uni-directional carousel system can be feasible. The items that customers order are retrieved from the bins of the carousel with batch size. The mathematical model to obtain the rotary travel distance and the retrieval lead time for picking batch size in an uni-directional carousel system are formulated. The rotary travel distance is the distance which the carousel system rotates for retrieval of all items included in a batch. The retrieval lead time is the time interval from retrieval starting time to retrieval finishing time for retrieval of all items included in a batch. The retrieval batch size that carousel system can be feasible is obtained. A numerical example is shown to explain the solution procedure.