Reorder Point (ROP

Supply Chain & Operations: The Mechanics of the Reorder Point (ROP)

The Reorder Point (ROP) is the foundational trigger mechanism in fixed-order-quantity inventory control systems (the continuous review (s, Q) policy):

• Continuous Review Principle: Inventory levels are continuously tracked in real time. The moment the inventory position drops to or below a batch of size Q (often the Economic Order Quantity) is immediately ordered.

• Effective Inventory Position vs. Physical On-Hand: Ordering decisions must never look solely at physical warehouse stock; they must evaluate \ to prevent accidental double-ordering while an existing shipment is in transit.

• Stockout Risk Window: The only time an inventory system is vulnerable to stockouts is during the replenishment lead-time window between placing a purchase order and receiving the goods at the loading dock.

How to Use This Calculator

Enter your Average Daily Sales / Demand (units/day) (e.g. 80), Supplier Replenishment Lead Time (days) (e.g. 10 (Time from PO creation to warehouse dock)), Safety Stock Buffer (units) (e.g. 200 (Buffer against stockouts)), Reorder Batch Size (EOQ / Q in units) (e.g. 500), Current Physical On-Hand Stock (units) (e.g. 320), On-Order / In-Transit Inventory (units) (e.g. 0 (Open Purchase Orders)), and Reserved / Allocated Customer Orders (units) (e.g. 40 (Orders placed but not yet shipped)). Results update instantly as you type — adjust any value to compare scenarios.

The Formula

Inventory Reorder Point (ROP) \& Inventory Position equations:

Lead Time Demand (LTD) = d × L

Reorder Point (ROP) = Lead Time Demand + Safety Stock = (d × L) + SS

Effective Inventory Position (IP) = On-Hand + On-Order - Allocated

Reorder Trigger Rule: Place Purchase Order if IP ROP

Runway Days to Reorder = (0, (IP - ROP) / (d))

Maximum Inventory Level = ROP + Q, Average Inventory = SS + (Q) / (2)

Worked Example

For example, managing an item with average sales \ units/day, supplier lead time \ days, safety stock \ units, batch size \ units, on-hand stock of 320 units, and 40 allocated units: Lead-time demand is \ units. Total Reorder Point is . Effective inventory position is . Because 280 1,000, an immediate replenishment purchase order of 500 units is triggered.

Understanding Your Result

Evaluate your result in the context of your specific use case and relevant industry standards or reference values. A single calculated result is a starting point — test two to three scenarios to understand how sensitive the outcome is to your key input assumptions. For high-stakes decisions, verify your result against a secondary source or subject-matter expert before acting.