What Is a Reorder Point? The Formula, a Real Example, and Where It Breaks for Ecommerce
A reorder point is the inventory level that triggers a new purchase order. The formula is simple. The part nobody tells you is where it stops working for Amazon FBA, Shopify, and multi-channel sellers.
Quick Answer
A reorder point (ROP) is the stock quantity at which you place a new order. The standard formula is Reorder Point = (Average Daily Sales × Lead Time in Days) + Safety Stock. For a SKU selling 10 units/day with a 30-day lead time and 150 units of safety stock, the reorder point is 450 units. When on-hand inventory drops to 450, you order.
The formula works cleanly for single-channel, single-warehouse businesses. It starts to lie the moment you add Amazon FBA’s hidden receiving window, a second sales channel like Shopify, or seasonal demand shifts. This post covers both the formula and where it breaks.
The reorder point formula
Every version of this formula you’ll find online reduces to the same thing:
Three inputs. That’s it. The complexity isn’t in the math — it’s in getting the three inputs right. Most stockouts happen not because someone got the formula wrong, but because one of the three inputs was stale, underestimated, or applied to the wrong inventory bucket.
- Average daily sales — your sales velocity for this SKU, measured in units per day. Use the last 30–90 days for stable SKUs. For seasonal SKUs heading into a peak, use a forward-looking estimate or a seasonality-adjusted velocity.
- Lead time — the number of days from placing the order to having sellable inventory on the shelf. This is where most ecommerce sellers get it wrong (more on that below).
- Safety stock — the buffer that protects you from demand spikes and late shipments. A simple version:
(Max Daily Sales - Average Daily Sales) × Max Lead Time. A better version uses the standard deviation of demand, but for brands under 500 SKUs the simple version works fine.
Calculate your reorder point (try it live)
Plug in your own SKU numbers below. The defaults are a real mid-velocity Amazon FBA product — a silicone kitchen spatula selling 12 units/day with an overseas supplier. Change any field and the reorder point recalculates instantly.
Reorder Point Calculator
When your sellable inventory drops to 924 units, place the order.
Notice what happens when you zero out the “FBA receiving” field — the reorder point drops by roughly 15–25%. That’s the gap most sellers miss. They calculate the reorder point using the factory quote, forget the receiving window, and stock out weeks before the next shipment arrives.
Now imagine doing this for 200 SKUs across two channels every Monday morning. That’s the point where a calculator isn’t enough and a real forecasting template or automated software starts earning its keep.
The hidden lead time nobody mentions: FBA receiving
If you sell on Amazon FBA, your lead time is not what the factory tells you. It’s the factory time plus shipping plus Amazon’s receiving and check-in window. That last piece — FBA receiving — adds 7 to 21 days depending on the season, the fulfillment center, and whether Amazon is running a backlog. During peak season (Q4), receiving times can stretch past 30 days.
Every textbook reorder point post on the internet treats lead time as a single number from the supplier. For ecommerce sellers shipping into FBA, that’s a dangerous simplification. Your real lead time includes every step from PO placement to the moment the units show as “Fulfillable” in Seller Central.
The same applies to Walmart WFS — receiving windows run 5–15 days and fluctuate with Walmart’s seasonal volume. If you sell on both, you need two different lead times feeding two different reorder points for the same SKU.
Multi-channel sellers need more than one reorder point per SKU
Here’s the part that breaks the textbook formula for most growing brands: if you sell the same SKU on Amazon FBA and through your own Shopify store (shipped from a warehouse or 3PL), you don’t have one inventory pool — you have two. FBA replenishes on a 45–90 day cycle from overseas. Your warehouse ships next-day to Shopify customers.
A single reorder point averaged across both channels will either overstock FBA or understock your warehouse. The operator answer is to run two reorder points per SKU:
- FBA reorder point — uses FBA-specific velocity and the full factory-to-FBA lead time (60–90 days for overseas)
- Warehouse reorder point — uses Shopify/direct velocity and the factory-to-warehouse lead time (often shorter if you hold buffer stock)
This is exactly the multi-channel forecasting approach that SKU Compass automates — separate velocity calculations per channel, separate lead times, one unified purchase order recommendation.
When the reorder point formula stops working
Seasonal demand shifts: The formula uses historical average velocity. If you’re heading into Q4, Prime Day, or Back-to-School and your sales are about to double, the average from the last 90 days will under-order. You need to manually bump the velocity input or use a seasonality-adjusted forecast.
New product launches: A SKU with 30 days of sales history has no reliable average. The formula will give you a number, but it’s fiction. Hold extra safety stock and review weekly until you have 90+ days of data.
Lumpy demand (B2B, wholesale): If a SKU gets one order a month for 500 units from a wholesaler plus 5 units/day from Amazon, the “average daily sales” is meaningless. The demand distribution isn’t smooth enough for a simple average to predict anything.
Bundle and kit components: If a component SKU feeds three different bundles, the reorder point needs to account for combined demand across all parent SKUs — not just the component’s own sales. Most sellers miss this until a bundle stocks out and they can’t figure out why.
Want to run the numbers yourself?
We built a free inventory forecasting Excel template that calculates reorder points using the days-of-coverage approach — the same method from this post, but with FBA/warehouse split, open PO handling, and configurable lead time and safety stock defaults. Grab it and be running in 15 minutes.
If you’ve outgrown spreadsheets and want reorder points calculated automatically across every channel every night, check out SKU Compass. It’s the tool we built after the spreadsheet stopped being enough.
Frequently asked questions
What is a reorder point in inventory management?
A reorder point (ROP) is the inventory quantity at which a new purchase order should be placed for a specific SKU. The standard formula is: Reorder Point = (Average Daily Sales × Lead Time in Days) + Safety Stock. When on-hand stock drops to this level, ordering immediately ensures new inventory arrives before you stock out.
How do you calculate a reorder point for Amazon FBA?
For Amazon FBA, the key difference is lead time. Your real lead time includes factory production + ocean freight + customs + FBA receiving and check-in (7–21 days, longer during Q4). Most sellers undercount lead time by ignoring FBA receiving, which leads to stockouts. Use the full factory-to-FBA-sellable timeline, not just the supplier’s quoted production time.
What is the difference between a reorder point and safety stock?
Safety stock is a buffer — extra units held to protect against unexpected demand spikes or late shipments. The reorder point is the total trigger level that includes both the expected lead-time demand AND the safety stock. Safety stock is a component inside the reorder point, not a separate concept. You can have safety stock without a formal reorder point, but you can’t have a useful reorder point without safety stock.
How does FBA receiving time affect your reorder point?
FBA receiving adds 7–21 days (sometimes 30+ during peak season) to your effective lead time. If your factory says “21 days” and shipping is 28 days, your real lead time to sellable FBA inventory is 56–70 days, not 49. Ignoring FBA receiving is the single most common reason Amazon sellers stock out even when they think they ordered on time.
Should you use different reorder points for each sales channel?
Yes, if you sell the same SKU on more than one channel (e.g., Amazon FBA and Shopify). Each channel has a different sales velocity and a different replenishment lead time. FBA might take 60–90 days to restock from overseas; your own warehouse might take 30. A single averaged reorder point will either overstock one channel or understock the other. Run separate reorder points per channel.
What happens if you set your reorder point too high or too low?
Too low: You stock out. On Amazon, a stockout kills your organic ranking and can take weeks to recover. On Shopify, it’s lost revenue and a bad customer experience. Too high: You tie up cash in inventory, pay higher storage fees (especially FBA aged inventory surcharges), and risk sitting on dead stock if demand shifts. The goal is to set it just right — which means reviewing it monthly, not once.
Does the reorder point formula work for seasonal products?
Not without adjustment. The standard formula uses average historical velocity, which smooths out seasonal peaks and valleys. If you sell 20 units/day in December and 5 units/day in March, the average of 12.5 will over-order in slow months and under-order in fast ones. For seasonal SKUs, replace the average with a forward-looking seasonal velocity estimate, or use a seasonality index that weights the upcoming period more heavily.