This study aimed to (1) analyze requirements, design, and develop a web-based inventory management system integrated with a point-of-sale (POS) system for small and medium-sized enterprises (SMEs); (2) apply Reorder Point (ROP) and Economic Order Quantity (EOQ) calculations to support purchasing planning and decision-making; and (3) evaluate the system in terms of inventory-data accuracy, operational speed, and inventory-related cost reduction. An engine parts shop was used as a case study.
         The system was implemented using PHP with the Laravel framework and a MySQL relational database. Requirements were elicited from the shop’s existing workflow through on-site observation, interviews with the owner and staff, and the review of product, sales, and historical purchasing records. The system comprises modules for inventory operations, POS transactions, dashboards and reporting, EOQ/ROP computation, average-cost computation, cost of goods sold (COGS) computation, and storage-area cost evaluation. System correctness was verified through unit and integration testing, and computational outputs (EOQ/ROP and cost calculations) were validated against spreadsheet-based reference calculations.
         Results show that the system supports POS-based sales transactions, automatic stock deduction after sales, stock-movement history logging, and dashboard-based monitoring. Across 20 product items, EOQ values ranged from 17 to 211 units per order and ROP values ranged from 1 to 4 units. A cost comparison indicates that the EOQ-based policy reduced the annual total inventory cost from 46,300 to 34,300 baht (25.92%), with reductions in both holding and ordering costs. The system also provides average cost, COGS, and storage-area cost metrics to support practical decision-making. As a limitation, ROP in this study is based on average demand and lead time without incorporating safety stock; therefore, performance under demand or lead-time variability should be further investigated.