Currently, the criteria for determining the optimal order quantity and the holding quantity to achieve maximum cost-effectiveness have not yet been applied in the inventory management of the factory in the case study. Therefore, to reduce the factory's costs and improve the efficiency of raw material management to meet demand, this research aims to study the improvement of inventory management in the factory by applying the theory of different order quantity methods and comparing them with the actual inventory costs from April 2022 to March 2023 using three heuristic methods: 1. Economic Order Quantity (EOQ) 2. The Periodic Order Quantity (POQ) method, as per the Silver-Meal approach, found that all three heuristic order size determination methods had lower inventory costs than the actual inventory costs. The method that provided the lowest cost for this case study was the POQ method, which reduced the factory's inventory costs by 79.59%. Additionally, the calculation mechanism of the POQ method is not complex and is easy to implement in practice.