Abstract: Batch sizing strategy in the manufacturing system has significant impacts on the production performance. Most researchers used some complicated techniques such as simulation, queuing theory, optimization model or complex system equations to find the optimal batch size. Applying those techniques to assess the effects of the change in the production factors such as demands and capacity on the process performance requires some customizations and is difficult. In this study, we proposed closed-form optimal batch size equations that can be easily used to assess the impact of changes in production volume. The number of batches from the optimal batch size equation may not be integer but they provide the lower bound on the process lead time.