A reorder point (ROP) is the inventory level at which a new replenishment order should be placed so that the goods arrive before existing stock runs out. In inventory management for e-commerce, it converts two facts, how fast a product sells and how long replenishment takes, into a single trigger quantity for each SKU.
The reorder point answers the question of when to order, not how much. It works together with safety stock, which protects against uncertainty, and with an order quantity rule, which decides the size of each purchase. Set correctly, it prevents both stockouts and unnecessary excess inventory.
How the reorder point is calculated
The standard reorder point formula is ROP = average daily demand × lead time in days + safety stock. The first part, called lead time demand, covers expected sales while the order is on its way; safety stock covers sales above that expectation and delays in delivery.
| Input | Definition | Typical data source |
|---|---|---|
| Average daily demand | Units sold per day over a representative period | Order history, demand forecast |
| Lead time | Days from placing an order to stock being sellable | Supplier records, receiving data |
| Safety stock | Buffer for demand and lead time variability | Statistical calculation or policy |
| Service level | Target probability of not running out per cycle | Business decision by SKU class |
Lead time must include every step until the product can be sold: supplier processing, transport, customs where relevant, receiving and putaway in the warehouse. Using only the supplier’s quoted production time is a common reason why calculated reorder points are too low.
A frequently used statistical approach sets safety stock = z × standard deviation of daily demand × square root of lead time, where z comes from the chosen service level. When lead times also vary, extended formulas add a term for lead time variability.
Service level and safety stock
The service level decides how much protection the reorder point provides. Higher service levels require disproportionately more safety stock, because each additional step of protection covers rarer demand peaks.
| Cycle service level | Approximate z value | Typical use |
|---|---|---|
| 90 percent | 1.28 | Low-margin or easily substituted items |
| 95 percent | 1.65 | Common default for standard products |
| 97.5 percent | 1.96 | Important items with loyal demand |
| 99 percent | 2.33 | Bestsellers and critical spare parts |
Moving from 95 to 99 percent raises safety stock by roughly 40 percent under the normal-distribution assumption. For this reason, experienced planners assign service levels by ABC classification: the few products that drive most revenue receive high protection, while the long tail receives less.
The cycle service level measures the chance of not running out during one replenishment cycle. It is different from the fill rate, which measures the share of demand served from stock; a product can show a modest cycle service level and still a high fill rate if shortages are small.
Reorder point versus related inventory concepts
The reorder point is often mixed up with neighboring inventory terms. Each concept answers a different planning question.
| Concept | Question it answers | Relation to the reorder point |
|---|---|---|
| Reorder point | When should a new order be placed | The trigger level itself |
| Safety stock | How much buffer protects against uncertainty | Included inside the reorder point |
| Economic order quantity | How many units to order at once | Decides order size after the trigger |
| Demand forecasting | How much will sell in the future | Supplies the demand input |
| Just-in-time inventory | How to minimize stock on hand | Requires short, reliable lead times and low reorder points |
| Backorder | What happens when stock is insufficient | The result of a reorder point set too low |
Two review methods use the reorder point differently. In continuous review, the system checks stock after every sale and orders as soon as the level falls to the reorder point. In periodic review, stock is checked on fixed days, so the protection must also cover the review interval, which raises the effective reorder level.
Applying reorder points in an online store
In e-commerce, reorder points are usually maintained in an inventory or warehouse management system that alerts buyers or creates draft purchase orders automatically. Their accuracy depends on clean, synchronized stock data across all sales channels.
- Use available stock for the trigger, meaning on hand minus reserved orders, plus stock already on order.
- Recalculate demand regularly, for example weekly for fast movers and monthly for slow movers.
- Adjust for seasonality and promotions rather than relying on a long flat average.
- Exclude periods of stockout from demand history, since zero sales then do not mean zero demand.
- Review supplier minimum order quantities, which can force larger orders than the formula suggests.
Common reorder point mistakes
Most reorder point failures come from static settings in a changing business. The table lists frequent symptoms with their causes and fixes.
| Symptom | Likely cause | Fix |
|---|---|---|
| Repeated stockouts | Lead time underestimated | Measure actual order-to-shelf time |
| Excess inventory | Demand average from an old peak | Use recent, seasonally adjusted demand |
| Stockouts during promotions | Promotion uplift ignored | Temporarily raise the reorder point |
| Orders triggered too late | Open orders or reservations ignored | Base trigger on inventory position |
| Dead stock in the long tail | Same service level for all SKUs | Differentiate service levels by class |
The idea in brief
A reorder point is the level at which it is time to buy more. It works like the fuel warning light in a car: the light comes on while there is still enough fuel to reach a gas station.
For a shop, the warning level is set so that the remaining stock lasts until the next delivery arrives. A small extra cushion covers busier days. It also helps when the supplier is late.
A practical scenario
An online pet supply store sells an average of 20 bags of a popular cat food per day, with a standard deviation of 6 bags. The supplier delivers in 7 days, including receiving at the warehouse, and the store targets a 95 percent cycle service level.
Lead time demand is 20 × 7 = 140 bags. Safety stock is 1.65 × 6 × √7, which is about 26 bags. The reorder point is therefore about 166 bags, so the inventory system creates a purchase order whenever available stock plus open orders falls to that level.
Before a planned holiday promotion that is expected to double sales for two weeks, the buyer temporarily raises the reorder point. The store keeps the product in stock throughout the campaign, and the outcome is no lost sales on its bestseller without a permanent increase in inventory.