INVENTORY CONTROL
How to Set a Reorder Point for Ecommerce Inventory
Convert sales velocity and replenishment lead time into an inventory trigger, with a defensible safety-stock buffer.
- Published
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DIRECT ANSWER
The calculation in one paragraph
Reorder point equals average demand during replenishment lead time plus safety stock. With daily units, use average daily demand × lead-time days + safety stock units. Trigger on inventory position—not only shelf quantity—when purchase orders, committed customer orders, backorders, and transfers materially affect what will be available.
Open the reorder point calculatorMatch demand rate and lead-time units
Average daily demand must be multiplied by total replenishment lead time in days. Count supplier processing, production, transit, customs where relevant, receiving, and put-away—not merely the carrier estimate. If demand is weekly, convert lead time to weeks or demand to days before multiplying.
Shopify presents the same base formula: daily sales velocity × lead time + safety stock. SAP reorder-point planning likewise treats the reorder level and safety stock as inventory-planning controls. The arithmetic is simple; reliable inputs and a clear trigger definition are the hard part.
Choose safety stock for uncertainty, not comfort
Safety stock covers variability in demand and lead time. A simple operational estimate is maximum lead-time demand minus average lead-time demand, but this is sensitive to rare extremes and short histories. A service-level model can be better when clean demand and lead-time distributions are available.
Segment the policy. High-margin, long-lead, or service-critical SKUs may justify more buffer; perishable, trend-sensitive, bulky, or cash-intensive SKUs may justify less. Record the target service level or risk judgment so the buffer can be reviewed instead of becoming permanent unexplained inventory.
Trigger on the inventory position that can satisfy demand
On-hand inventory alone can mislead. A useful inventory position is on hand + confirmed inbound − committed outbound − backorders, adjusted for transfers and unusable stock. Avoid double counting inbound units that are already allocated. Apply the reorder point separately by fulfillment location when stock cannot move fast enough between locations.
A reorder point tells you when to act, not how much to buy. Order quantity must also respect minimums, case packs, freight economics, shelf life, storage capacity, working capital, and the demand expected through the next review cycle. Recalculate after promotions, assortment changes, supplier performance shifts, or season changes.
WORKED EXAMPLE
SKU reorder example
- Average demand: 7 units/day
- Replenishment lead time: 18 days
- Safety stock: 35 units
- Inventory position today: 156 units
(7 × 18) + 35161-unit reorder pointBecause inventory position is 156, the reorder is due. The purchase quantity is a separate decision; it may need to be rounded to the supplier’s 12-unit case pack.
Inventory inputs and common traps
| Input | Include | Common trap |
|---|---|---|
| Demand rate | Units actually demanded | Treating stockout days as zero demand |
| Lead time | Order release through available-to-promise | Using transit time only |
| Safety stock | Demand and lead-time uncertainty | Adding an arbitrary percentage forever |
| Inventory position | On hand + inbound − commitments | Triggering from on-hand alone |
A practical workflow
- Select the SKU, location, demand history, and review cadence.
- Calculate representative daily demand without mixing stockout days into ordinary low demand.
- Measure end-to-end lead time, including receiving and put-away.
- Set a documented safety-stock method and calculate the reorder point.
- Apply the trigger to inventory position, then round the purchase quantity to operational constraints.
EDGE CASES
Frequently asked questions
How often should reorder points be updated?
Review on a cadence appropriate to volatility and whenever demand, supplier lead time, service target, or assortment changes materially. Seasonal products may need dated policies rather than one annual average.
What if demand is intermittent?
A simple average can overstate most periods and understate rare demand. Use a longer horizon, separate event demand, consider order-up-to or periodic review methods, and apply business judgment to service-critical items.
Is safety stock the same as reorder point?
No. Safety stock is the uncertainty buffer. Reorder point includes expected lead-time demand plus that buffer.