SOLUTIONS
We keep running out of our best sellers: how to prevent stockouts
A stockout loses today's sale and often the customer. Here is why best sellers run out, what it costs and the fix, step by step, whether you do it yourself or we run it.
Stock Flow $1,500 to $3,500 · included in Ops Desk Growth at $2,499 a month
Best sellers run out when reorders happen from memory instead of from a rule. To prevent stockouts on best sellers, give every top SKU a reorder point built on its real lead time, track each purchase order to the day it can be sold and get an alert before cover runs low. You can start this week with a spreadsheet.
What it looks like when best sellers keep running out
"We keep running out of our best sellers."
Someone notices a top SKU is low, a reorder goes out that afternoon and the supplier's 35-day production run lands in the middle of the promotion it was meant to feed. The lead time lives in one person's head. Meanwhile slow SKUs sit in storage, paid for with the cash the best seller needed.
The quieter version is the variant. A hoodie sells in six sizes and four colors, and the medium black runs out first. The listing still shows in stock, so nobody panics, but the shopper who wanted medium black leaves.
What a stockout costs
A stockout costs the sale, and often more. In a worldwide study of shoppers who found an item out of stock, 31% went to another store, 26% switched to a different brand and 9% bought nothing at all (FMI, GMA and CIES study, Oct 14, 2002). Researchers Tom Gruen and Daniel Corsten later found that out-of-stock rates for online purchases were double those in stores (UNH Today, Mar 20, 2019).
Running thin at Amazon costs money before you run out. Amazon's low-inventory-level fee applies when both your 30-day and 90-day historical days of supply fall below 28 days. Our low-inventory fee guide explains the exemptions.
Price yours. Daily revenue of the SKU × days out of stock × 66% × your contribution margin. The 66% is the share of shoppers in the 2002 study who bought elsewhere, switched brand or bought nothing. It is a grocery study, so treat it as a starting assumption and replace it with your own data once you have a few stockouts logged.
| Input | Value |
|---|---|
| Daily revenue on the SKU | $1,200 |
| Days out of stock | 10 |
| Share of demand lost | 66% |
| Contribution margin | 30% |
| Contribution lost in one stockout | $1,200 × 10 × 66% × 30% = $2,376 |
Illustrative · sample figures
Illustrative · sample figures
Why best sellers run out: five causes
Most stockouts trace back to one of five causes. Run the check before you change anything, because a lead time problem and a cash problem need different fixes.
| Cause | How to check | What fixes it |
|---|---|---|
| No reorder rule | Ask who decides when to reorder, and on what number | A reorder point per SKU and variant |
| Lead time measured to the dock | Compare the PO date with the day stock was available to sell | Lead time counted to available-to-sell |
| Purchase orders not tracked | Can you list every open PO with an arrival date right now? | A weekly PO and inbound review |
| Stock in the wrong place | FBA empty while your 3PL holds units, or the reverse | Cover targets per location |
| Cash timing | Reorders delayed because cash is tied up in slow stock | A 13-week cash view (Profit and Cash Pulse) |
The second cause is the easiest to miss. A supplier quotes 30 days. Freight adds 25 and customs and drayage add 6. FBA receiving takes another 10 in a busy month. The real lead time is 71 days, and a reorder point built on 30 runs out every time.
How to prevent stockouts on best sellers, step by step
The fix is a short routine that runs every week, not a one-off project. These are the seven steps in the order we set them up.
- 01List the SKUs that carry your revenue, down to the variant.
These get the tightest rules.
- 02Measure full lead time per supplier.
Production, freight, customs and receiving at FBA, WFS or your 3PL, counted to the day the units can be sold.
- 03Set a reorder point and safety stock for each.
The reorder point is average daily sales × lead time in days + safety stock. The full formula and a worked example are on our inventory and 3PL page, and the reorder point calculator does the arithmetic.
- 04Set cover targets per location.
Keep FBA above 28 days of supply where the economics make sense, and decide how much sits at your 3PL as a backstop.
- 05Track every purchase order and inbound shipment to arrival, weekly.
An open PO without an expected sellable date is a stockout waiting to happen.
- 06Alert before cover drops below target, not after.
The alert should fire when there is still a full lead time left to act.
- 07Run a short post-mortem on every stockout.
SKU, days out, cause from the table above, fix and owner. Five lines, filed in one place.
Too thin and too thick: aim for a cover range
Ordering more is not the fix, because overstock has its own price. Walmart WFS charges $0.75 per cubic foot a month for standard storage, plus $1.50 per cubic foot a month in October to December for units stored more than 30 days. From Jun 30, 2026, units older than 365 days cost $2.25 per cubic foot a month to day 450 and $7.50 after that (Walmart WFS fees, updated May 27, 2026). Our WFS fees guide has the full table.
Peak season squeezes both ends. Amazon's 2026 holiday peak fee averages $0.32 a unit on shipments from Oct 15, 2026 to Jan 14, 2027. Amazon also advises getting inventory in by October for Black Friday and Cyber Monday delivery speeds (Retail Dive, Jul 14, 2026). So set a cover range per SKU, with a floor and a ceiling, and review it every week.
What to do yourself this week
You can find your stockout risk in an afternoon:
- Export the last 90 days of sales by SKU and channel.
- Write down the real lead time for your top 10 SKUs, counted to available-to-sell.
- Calculate days of cover for each: (available units + inbound units) ÷ average daily sales. For example, 900 available plus 600 inbound, selling 40 a day, is 37.5 days of cover (Illustrative · sample figures).
- List every open PO with its expected arrival date.
- Put a 30-minute stock review on the same day each week, with one owner.
When it is not a reorder problem
Sometimes the reorder rule is fine and something else broke. A supplier at capacity needs a second source or a longer lead time in the plan. A promotion nobody added to the forecast needs a rule that promotions go through the stock review first. Cash that cannot fund the order needs a decision about slow stock. The post-mortem tells you which one you have.
If units sell on two channels at once, that is overselling, with its own fix: see how to prevent overselling. The other seven problems we see most often, from 3PL errors to shrinking margin, are listed on our common ecommerce operations problems page.
How we run it
Stock Flow is the Relay Module that builds this in your accounts. It sets up one stock count across Amazon FBA, Walmart WFS, Shopify and your 3PL, reorder points per SKU, purchase order drafts and alerts. It costs $1,500 to $3,500, fixed in writing before the build, and takes 1 to 4 weeks. You pay 50% at the start and 50% at acceptance, and anything that fails acceptance is fixed free for 60 days. See Stock Flow for the build spec.
On Ops Desk Growth ($2,499 a month), Stock Flow is included and our team runs it: stock and inbound checked every business day, the reorder plan updated weekly and purchase orders drafted for you. You approve every PO. We never place an order with your supplier on our own.
Founding offer: 20% off launch fees, add-ons and the Foundation, plus 20% off monthly plans for the first 6 months, for our first 25 clients or until Jan 31, 2027, whichever comes first. Full terms are on pricing.
How to measure it
Track four numbers, each with the period stated: stockout days on top SKUs per quarter, days of cover by location each week, inbound shipments that arrived late and contribution lost to stockouts, estimated with the formula above. We report these numbers. We do not promise them, because supplier delays and demand spikes are real. The free Profit Leak Scan gives you the baseline.
What does it cost to fix stockouts?
Stock Flow is $1,500 to $3,500 as a one-time build, fixed in writing before we start, or included in Ops Desk Growth at $2,499 a month with our team running it daily. To see your risk before spending anything, the free Profit Leak Scan shows stockout and overstock risk within 48 hours of working read-only access.
How much stock should we keep at Amazon?
Enough to stay above 28 days of supply where the margin supports it, because the low-inventory-level fee applies when both your 30-day and 90-day historical days of supply fall below 28 days. Above that, storage cost and cash set the limit, so many brands keep backup units at a 3PL.
What is a reorder point?
It is the stock level that triggers the next purchase order: average daily sales × full lead time in days, plus safety stock for demand swings and late shipments. Count lead time to the day units are available to sell, not the day they leave the factory.
Should we just order more?
No. Overstock trades a stockout for storage and aged-stock fees, and it ties up the cash your next reorder needs. On short-dated or seasonal products it adds expiry and markdown risk too. Set a cover range per SKU and location instead, with a floor and a ceiling, and review it weekly.
Will you place orders with our suppliers?
No. We draft purchase orders with quantities, dates and the reasoning behind them, and you approve every one. Supplier terms, payment and the relationship stay with you. Product sourcing is part of Full Relay, our managed store service, not Stock Flow.
How fast can we see our stockout risk?
The Profit Leak Scan reports it within 48 hours of working read-only access, with stockout and overstock risk, aged and low-inventory fees and a fix list ranked by dollar impact. It arrives by email, and you keep it whether or not you hire us.
Stockouts stop when the reorder decision has a number, an owner and a weekly slot in the calendar.
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Ecomrelays LLC is a Texas company. Part of our team works outside the US, as one managed team with a named US-facing lead. 597 Mountcastle Dr, Rockwall, TX 75087. +1 (469) 331-3129.
Sources4 sources
- 01fmi.orgFMI, GMA and CIES out-of-stock study, Oct 14, 2002; checked Oct 20, 2026Primary
- 02unh.eduUNH Today, Gruen and Corsten, Mar 20, 2019; checked Oct 20, 2026Primary
- 03marketplacelearn.walmart.comWalmart WFS fees, last updated May 27, 2026; checked Oct 20, 2026Primary
- 04retaildive.comAmazon 2026 holiday peak fee, Jul 14, 2026; checked Oct 20, 2026Primary
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