Reorder point (ROP)
The reorder point (ROP) marks the inventory level that triggers a replenishment order for an item. The standard formula in plain language: average daily demand multiplied by lead time in days, plus safety stock. If you use 40 units a day, lead time is 10 days, and you hold 120 units of safety stock, you reorder when inventory position reaches 520.
Examples
Setting the point: A machine shop uses 25 brackets a day, the supplier ships in 12 days, and safety stock is 75 units. ROP = 25 × 12 + 75 = 375. When inventory position hits 375, the buyer releases the standard 1,500-piece order.
Lead-time drift: Two quarters later the same supplier quotes 18 days, but the ROP still says 375. Expected demand during lead time is now 450, so the part runs out nearly every cycle until the point is reset to 25 × 18 + 75 = 525.
Position, not shelf count: The shelf shows 300, below the 375 trigger, but 1,500 pieces are already on order. Inventory position is 1,800, so no new order fires. A system triggering on shelf count alone would have ordered twice.
Definition
ROP answers when to order; economic order quantity answers how much. Together they define the classic continuous-review policy: when inventory position falls to the reorder point, release an order of the standard quantity.
The formula has two parts. Demand during lead time covers expected consumption while the replenishment is in transit; safety stock covers the bad weeks when demand spikes or the shipment runs late. The trigger must be computed on inventory position (on hand plus on order minus backorders), not shelf count alone, or the system double-orders.
ROPs fail quietly. The most common mode is lead-time drift: the parameter says 4 weeks because that is what the supplier quoted at award, the supplier now ships in 7, and the part stocks out every cycle while everyone blames demand. Recalculate whenever demand or lead time moves, review parameters at least quarterly, and audit the items your inventory management system keeps expediting.
Frequently asked questions
What is a reorder point in inventory management?
A reorder point is the inventory level that triggers a replenishment order for an item. When inventory position falls to that level, the buyer releases an order of the standard quantity, so the reorder point answers when to order while economic order quantity answers how much.
How do you calculate a reorder point?
The standard formula is average daily demand multiplied by lead time in days, plus safety stock. If a shop uses 25 brackets a day, the supplier ships in 12 days, and safety stock is 75 units, the reorder point is 25 times 12 plus 75, or 375 units.
Why should a reorder point trigger on inventory position instead of shelf count?
Inventory position is on-hand stock plus open orders minus backorders, and computing the trigger on anything less causes double-ordering. If the shelf shows 300 against a 375 trigger but 1,500 pieces are already on order, position is 1,800 and no new order should fire. A system triggering on shelf count alone would have ordered twice.
Why do reorder points fail over time?
The most common failure is lead-time drift: the parameter still says 4 weeks because that is what the supplier quoted at award, the supplier now ships in 7, and the part stocks out every cycle while everyone blames demand. Recalculate whenever demand or lead time moves, review parameters at least quarterly, and audit the items the system keeps expediting.
What role does safety stock play in the reorder point?
The demand-during-lead-time portion of the formula covers expected consumption while the replenishment is in transit, and safety stock covers the bad weeks when demand spikes or the shipment runs late. Without that buffer, any variability in demand or delivery timing turns directly into stockouts.
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