Section 01
The Reorder Point Formula
ROP = (Average Daily Usage × Lead Time in Days) + Safety Stock. The formula has stayed the same for decades because it works. Average daily usage is how many units you sell, consume, or install per day. Lead time is how long it takes from placing an order to having usable stock on the shelf, including supplier processing, shipping, and receiving. Safety stock is the buffer that protects you when usage spikes or the supplier slips. The math is multiplication, not optimization. That's the point. The formula tells you when to reorder, not how much. Order quantity is a separate decision driven by case packs, supplier minimums, carrying cost, and storage. The two get confused constantly. Reorder point answers: at what shelf level should the next order go out? Order quantity answers: how many should be in that order? Keep them separate.
Section 02
A Worked Example A Plumbing Shop Can Use
Tuesday morning. A plumbing supply shop sells 8 of a half-inch copper elbow per day on average. The supplier ships in 3 days. The shop wants 5 units of safety stock to absorb a busy Friday or a delayed truck. ROP = (8 × 3) + 5 = 29. When the shelf hits 29 elbows, an order goes out. The shop typically orders cases of 50, so 50 units arrive 3 days later. On the day they arrive the shelf sits at roughly 29 - (8 × 3) = 5 units, which is the safety stock. Stock peaks at 55, dips to 5, replenishes to 55, repeats. If usage rises to 12 per day during a heatwave, the shop hits the reorder point faster (every 3-4 days instead of every 5-6) and the cycle compresses naturally. If lead time slips to 5 days because of a supplier issue, bump the reorder point to (8 × 5) + 5 = 45 until the supplier recovers. The formula updates as inputs change.
Section 03
How To Calculate Each Input Honestly
Average daily usage is best measured over the last 60 to 90 days. Anything shorter is noisy. Anything longer hides recent shifts. If your business is seasonal, calculate usage by season, not by trailing average. An irrigation supplier should not use January usage to set April reorder points. Lead time is more often misjudged than miscalculated. Operators measure lead time from when the supplier confirms the order, not from when the order leaves the building. The right number is the calendar days between you deciding to reorder and the stock being countable on the shelf. That includes approval delays inside your own team. If a PO sits in someone's email for two days, that's two days of lead time. Safety stock gets its own glossary entry; for now, a working rule is one to two days of average usage for stable items, three to five days for items where usage varies week to week.
Section 04
Why Reorder Points Should Vary By Location
A single reorder point across multiple locations is one of the most common configuration mistakes in small business inventory. A central warehouse needs a higher reorder point than a service truck because the warehouse fills more demand. A retail store with daily walk-in traffic needs a different rule than a backroom that resupplies once a week. Set the rule per location based on that location's usage and lead time, even if the supplier is the same. The lead time at a service truck includes the time to do the internal transfer from the warehouse, four hours or two days depending on the dispatch process. A shop running the same threshold everywhere ends up with overstock at low-velocity locations and stockouts at high-velocity ones. Tools that support per-location reorder rules let you keep the formula simple and let the location absorb the variability.
Section 05
When To Use AI-Assisted Suggestions
Manually maintaining reorder points across hundreds of SKUs is where this work breaks down. Usage shifts week to week. Lead times drift. Seasonality changes the inputs faster than anyone has time to update. AI can help here without taking control. The right pattern is suggestion, not automation: the system watches usage and lead time, notices that a SKU's reorder point looks low or high relative to recent history, and proposes a new threshold for someone to approve. The system never silently changes a threshold or places an order on its own. The buyer reviews the suggestion, accepts or edits it, and the change gets logged.
Section 06
Common Reorder Point Mistakes
Setting a reorder point on every SKU. Most teams have twenty to fifty items where a stockout actually causes pain. Set rules on those, leave the rest manual, tune the long tail later. Ignoring lead time variability. If the supplier is reliable on average but occasionally slips by a week, the average lead time understates risk. Use a slightly conservative number or hold more safety stock for those vendors. Letting the reorder point become permanent. Reorder points should be reviewed quarterly, especially after a vendor change, a price change, or a usage shift. And the most expensive one: treating the reorder point as the order quantity. The reorder point answers when. The order quantity answers how many. Confuse them and you end up with either constant tiny orders or rare oversized ones.