Inventory management in a company is an important factor in determining a company’s competitive success. PT. Siantar Top Tbk is a company that produces snacks, with biscuits being one of its main products. There is always a fluctuating and unexpected demand for biscuit products at this company, resulting in the company’s inability to meet the needs of products based on orders. This is due to a shortage of raw materials, which is the main factor in the production of biscuits. To achieve optimum inventory costs, it is necessary to formulate a formula related to how inventory is in accordance with the probabilistic pattern of raw material demand. The results of this study, which aims to determine the probabilistic inventory of biscuit raw materials for the Q model and then compare the total cost of biscuit raw material inventory for the Q-back order and Q-lost sales models, will be explained in this paper.