Efficient inventory management is crucial for businesses of all sizes. It enables companies to maintain optimal stock levels, fulfill customer orders promptly, and minimize holding costs. One inventory management approach gaining popularity is the perpetual inventory system. In this blog post, we will delve into the intricacies of the perpetual inventory system, exploring its definition, comparing it with the periodic inventory system, and evaluating its advantages and disadvantages. Additionally, we will discuss various perpetual inventory methods and highlight essential formulas used in this system. Let’s unlock the potential of perpetual inventory management.

What Is a Perpetual Inventory System?

The perpetual inventory system is an automated tracking and managing of real-time inventory. It relies on advanced software and technology to continuously monitor inventory levels, recording every transaction related to stock, such as purchases, sales, returns, and adjustments. This system provides up-to-date and accurate information about the quantity and value of each item in stock. As a result, it offers businesses enhanced visibility into their inventory, allowing for better decision-making and improved operational efficiency.

Perpetual vs. Periodic Inventory

To understand the benefits of the perpetual inventory system, it is essential to compare it with the periodic inventory system, a more traditional approach.

Periodic Inventory System: In a periodic inventory system, physical counts of inventory are conducted periodically, typically on a monthly, quarterly, or annual basis. The cost of goods sold (COGS) and the ending inventory value are determined through calculations based on the opening inventory, purchases, and physical counts. This method is simpler and requires fewer resources; however, it lacks real-time inventory tracking, making it less accurate and prone to discrepancies.

Perpetual Inventory System: In contrast, the perpetual inventory system provides continuous tracking of inventory levels. Each transaction, such as sales, purchases, or returns, is immediately recorded in the inventory management software, ensuring real-time updates. This system allows for accurate and timely information about stock levels, facilitates quick decision-making, reduces the risk of stockouts or overstocks, and enhances customer satisfaction.

Advantages and Disadvantages of Perpetual Inventory

Like any inventory management system, the perpetual inventory system has its own set of advantages and disadvantages:


  • Real-time inventory visibility: The perpetual inventory system offers instant access to accurate stock information, enabling businesses to make informed decisions regarding purchasing, production, and sales strategies.
  • Enhanced accuracy: By automatically recording every inventory-related transaction, the perpetual inventory system minimizes human errors and ensures precise inventory valuation and control.
  • Timely reorder alerts: With real-time data, businesses can set up automatic reorder points, triggering alerts when stock levels fall below a specified threshold. This helps prevent stockouts and maintain adequate inventory levels.
  • Reduced carrying costs: By efficiently managing inventory levels, the perpetual inventory system helps minimize carrying costs associated with an excess or obsolete stock.


  • Implementation costs: Implementing a perpetual inventory system requires investment in inventory management software, hardware, and staff training. This initial cost can be a barrier for small businesses.
  • Technical complexity: Utilizing advanced software and technology, the perpetual inventory system may require technical expertise to set up and maintain, making it challenging for some businesses to adopt.
  • Potential for data inaccuracies: While the perpetual inventory system reduces the risk of human error, technical glitches or system malfunctions can still lead to data inaccuracies if not monitored and addressed promptly.

Perpetual Inventory Methods

The perpetual inventory system operates by continuously updating inventory counts in real-time as items are bought and sold. Each sale triggers two double-entry transactions. The first transaction records the sale as a debit to accounts receivable and a credit to sales. The second transaction records the cost of goods sold (COGS) as a debit and decreases the inventory count with a credit.

Here are the steps involved in the perpetual inventory system:

  • Sales are recorded: Whenever an inventory item is sold, the point-of-sale system updates the inventory levels by reducing the count. This is typically done through barcode scanners, which expedite the process. For example, if a customer purchases four widgets, the system immediately decreases the inventory count by four units for that SKU. This sale triggers two accounting journal entries: one records the revenue credited and the other records the debit to either cash or accounts receivable.
  • COGS is updated: The perpetual inventory system automatically updates the COGS for each item purchased or sold. COGS includes expenses directly related to the production of inventory, such as overheads, raw materials, warehouse costs, and labor costs. If the COGS for each widget is $3.00, this amount is added to the overall COGS. Two more accounting journal entries are made: a debit is recorded against COGS on the income statement, and the same amount is credited to inventory on the balance sheet.
  • Replenishment levels are updated: Using historical sales and inventory data, the perpetual inventory system adjusts reorder points to maintain optimal inventory levels. Reorder points are frequently recalculated to account for fluctuations in demand. For example, if sales of a particular widget increased in the second quarter based on historical data, the system would automatically increase the reorder threshold to ensure sufficient inventory during that period.
  • Purchase orders are generated: When an inventory item hits its reorder point, the perpetual inventory system automatically generates purchase orders. These purchase orders are sent to the supplier, eliminating the need for manual ordering. For instance, if the system recognizes that only 50 SKUs of a specific widget are left in stock, it will generate a purchase order for 500 more units and send it to the supplier. The accounting journal entries are based on the quantity and cost of the purchased goods.
  • Receipt reconciliation: Upon receiving the inventory in the warehouse, the goods are scanned and reconciled in the perpetual inventory system. This ensures accurate tracking of the newly received items. Warehouse employees typically scan the items using warehouse management software, updating the inventory count in real-time. For example, if a shipment of 500 widgets arrives, each unit is scanned upon receipt, and the inventory count for that SKU increases by one for each scan.

By following these steps, the perpetual inventory system maintains an accurate and up-to-date view of inventory levels, enabling businesses to make informed decisions regarding sales, purchasing, and replenishment.

Perpetual Inventory Formulas

Certain formulas are essential in the perpetual inventory system to determine accurate inventory valuations and control:

  • Cost of Goods Sold (COGS)

COGS = Opening Inventory + Purchases – Closing Inventory

  • Average Cost

Average Cost = (Beginning Inventory + Purchases) / (Beginning Quantity + Purchases Quantity)

  • Inventory Turnover

Inventory Turnover = COGS / Average Inventory


Accurate and efficient inventory management is vital for success in today’s fast-paced business environment. The perpetual inventory system offers businesses a powerful tool to streamline their inventory control processes, providing real-time visibility, accurate valuations, and better decision-making capabilities. While there are initial implementation costs and technical complexities, the advantages of the perpetual inventory system, such as improved accuracy, reduced carrying costs, and enhanced customer satisfaction, make it a worthwhile investment. By leveraging perpetual inventory methods and formulas, businesses can optimize their inventory management practices and gain a competitive edge in the market. Embrace the perpetual inventory system and unlock the potential for enhanced operational efficiency and profitability.