Reorder Point (ROP) is the inventory level at which a new purchase order must be placed to avoid running out of stock before the next shipment arrives. It's the trigger that connects daily inventory tracking to replenishment action.

The reorder point formula

Reorder Point = (Average Daily Demand × Lead Time in Days) + Safety Stock

The intuition: when stock drops to a level just sufficient to cover demand during the lead time, plus a buffer for variability, place the next PO. Drop below that level and the brand risks stockout before the new shipment lands.

Worked example: an SKU sells 20 units per day, the supplier's lead time is 30 days, and safety stock is 100 units. ROP = (20 × 30) + 100 = 700 units. When inventory hits 700, place a PO.

Why reorder point matters

Without a defined ROP, replenishment becomes reactive: someone notices stock is low, scrambles to place a PO, and the brand often runs out before the shipment lands. With a defined ROP, replenishment becomes systematic: a clear trigger, a pre-calculated order quantity (typically EOQ or a multiple of MOQ), and predictable cash flow.

Reorder point vs. order quantity

ROP and order quantity are two halves of a complete replenishment policy:

Reorder point answers when to order.
Economic Order Quantity (EOQ) answers how much to order.

Most modern inventory tools combine the two into a continuous-review policy: monitor inventory in real time, trigger a PO at ROP, order EOQ (or the supplier MOQ if higher).

Setting and adjusting ROP

Use real lead time, not quoted lead time: suppliers tend to quote optimistic lead times. Track actuals over the past 6–12 months and use the realistic figure.
Refresh as demand shifts: a SKU that's selling 30 units/day instead of 20 needs an ROP recalculation, otherwise the trigger fires too late.
Adjust for known events: seasonal peaks, planned promotions, and product-launch cannibalization all shift demand temporarily — ROP should account for these in advance, not be surprised by them.
Account for cash constraints: the textbook ROP assumes the brand always has cash to place the PO. In reality, cash flow timing means some POs need to be placed earlier than ROP suggests, with the inventory carry-cost absorbed.

Common reorder point mistakes

Using average demand for SKUs with high variability: a SKU that swings between 5 and 50 units/day shouldn't have its ROP based on the average alone — the safety stock component needs to absorb that variability.
Not differentiating between hero and long-tail SKUs: hero SKUs warrant tighter ROP discipline and more frequent recalculation; long-tail SKUs can use simpler heuristics.
Treating ROP as static: demand and lead time both change. ROP should be reviewed quarterly at minimum.
Ignoring incoming POs already in transit: "inventory at ROP" means total inventory position (on-hand + on-order), not just on-hand. Placing a second PO when one is already in transit produces overstock.

The formula for reorder point is simple; keeping the inputs accurate at scale is the hard part. Lead time history, on-order quantities, and multi-location on-hand data usually live in a separate inventory or ERP system rather than Shopify itself, and if that data isn't synced cleanly, the ROP calculation is working from stale numbers before it even runs — the kind of data-mapping problem Shopify ERP services exist to fix. Once the inputs are trustworthy, the trigger itself should fire without a person watching a dashboard; PO-draft creation or alerts built with Shopify automation tools turn ROP from a number someone has to remember to check into a rule that runs on its own.

The reorder point formula is only as good as the demand estimate feeding it, and getting average daily demand right for a seasonal or volatile SKU is a forecasting problem rather than an arithmetic one — Shopify inventory forecasting covers how to build that estimate instead of anchoring it to a flat historical average.