Reorder Point Formula and Min Max Inventory Levels

The reorder point is the stock level at which you place the next order. Above it, wait; at or below it, order. It is the single most consequential number in day-to-day inventory work, and the formula fits on a napkin. The catch is not the arithmetic. The catch is that the inputs move and the napkin does not.

The formula

reorder point = (average daily demand × lead time in days) + safety stock

The first term is the stock you expect to sell while the replenishment order travels. The second is the buffer for non-average weeks, covered in depth in the safety stock guide. Together they answer: "if I order now, will I still be selling when it arrives?"

Worked example

A boutique sells 3 pairs of its core jeans a day. The supplier needs 21 days. Safety stock, computed at a 95 percent service level from the demand data, is 18 pairs.

reorder point = (3 × 21) + 18 = 63 + 18 = 81 pairs

When available stock (on hand plus already on order, minus reservations) touches 81, the next order goes out. How much to order is a separate question: enough to cover your planning horizon plus safety stock, minus what you will have left; our purchase order page walks the quantity math.

The three quiet failure modes

1. Stale inputs

The jeans start selling 5 a day after an influencer moment. The reorder point should now be (5 × 21) + buffer ≈ 130, but the system still says 81, so the order goes out two weeks late and the stockout arrives with receipts. Static reorder points rot at the speed your demand changes. Whoever recomputes them, an analyst on a schedule or software continuously, is your real inventory system.

2. Counting on hand instead of available

Sixty units on the shelf, forty already promised to open orders: the honest number is twenty, and the reorder point should trigger on it. Multichannel sellers hit this hardest; it is half of what ecommerce inventory management means.

3. Believing the quoted lead time

The supplier says 14 days; the last six deliveries averaged 19. Use 19. Track quoted versus actual per supplier and feed the real number into both terms of the formula.

Min/max inventory levels

A min/max inventory system sets two numbers per SKU: a minimum that triggers the reorder and a maximum you top back up to. The minimum is the reorder point above. The order quantity is simply the maximum minus available stock at the moment you order. It answers "when" and "how much" in one rule, which is why shelf-tag, two-bin and most ERP replenishment screens are built around it.

The maximum controls your carrying cost. Set it as the minimum plus the quantity you want to bring in each cycle, usually a number of days of supply or an economic order quantity that balances ordering cost against holding cost:

maximum = minimum + reorder quantity

Take a SKU that sells 5 units a day from a supplier that delivers in 4 days, with 20 units of safety stock, and say you want 20 days of supply per order.

When available stock falls to 40, you order up to 140. If a late delivery means you are at 31 when the order goes out, you order 109, not 100. That self-correcting quantity is the main advantage over a plain reorder point with a fixed order size.

Where min/max breaks

The weakness is the same as the reorder point's, doubled: two numbers per SKU that are usually typed once and left alone while demand, lead times and seasonality move. A max that made sense in spring can be twice what you need in autumn, so the fading lines overstock while the ones that took off run short. Min/max is fine for stable C items. For anything with a trend, the levels have to be recalculated from recent per-SKU sales, and that only works if the demand data feeding them is clean, so it pays to trust the numbers underneath before you automate decisions on top of them.

What is the difference between reorder point and safety stock?

Safety stock is the buffer you never plan to sell; the reorder point is the trigger level that includes that buffer plus the stock you expect to sell during the lead time. In the jeans example, safety stock is 18 pairs and the reorder point is 81. Change the service level and both move; change the lead time and only the reorder point moves.

Should the reorder point include stock on order?

Yes. Compare the reorder point with inventory position: on hand, plus open purchase orders, minus units already promised to customers. Triggering on shelf stock alone double-orders while a shipment is in transit, which is the most common reason small brands end up with two deliveries of the same SKU in one month.

Doing this for a thousand SKUs

The formula is trivial once. It is a job when it is per SKU, per location, monthly, with lead times per supplier, safety stock per service level and a min and max for every item. Storekeeper keeps those levels live: it reforecasts each SKU, resets the reorder point and order-up-to level, and drafts the order, which is what inventory replenishment software is for. That maintenance burden, not the math, is why reorder automation exists as a category. Our live stock scan shows the automated version: paste rows, get verdicts with the reasoning attached, including the reorder quantities the formula implies.