This paper develops an approximate method for predicting the service levels of a one-warehouse/N-identical retailer system operating under (Q, r) replenishment policy. The interarrival times of customer orders are assumed to be independent, and identically Erlang distributed. The service level of the retailer is formulated based on the convex combination of two types of events, viz. the warehouse has stock when the retailer orders, and the warehouse is out of stock when the retailer orders. The latter event gives rise to additional delay time the retailer has to wait for replenishment. Probability approximations are used to derive the distribution of the delay time. The special case of exponential interarrival times is considered. The proposed model performs relatively better than other existing heuristics when the number of orders placed at the warehouse increases.
