Stockout Days: What They Are and Why They Matter on FBA | Inventory Hero
·5 min readStockouts
Stockout Days: What They Are and Why They Matter on FBA
Stockout days are the days a SKU was unavailable. Why they quietly distort your velocity and IPI, how to find them, and how to exclude them from your math.
Stockout days are the days a SKU was not available to purchase. That includes days you were fully out of inventory and days the listing was live but unsellable, for example stranded, suppressed, or unfulfillable inventory. They are the days that generated zero sales not because demand was zero but because there was nothing to buy.
Why do stockout days matter for velocity?
Because if you average sales over a period that includes stockout days, the zero-sales days drag the average down and understate how fast the SKU actually sells. That understated velocity makes your days of supply read longer than it is, so you reorder late and risk running out again. Exclude stockout days from the denominator so velocity reflects real demand.
T. Brian Jones is co-founder and CTO of Inventory Hero. He leads the engineering behind its Amazon data pipeline, demand forecasting, and the AI platform that lets sellers talk to their live inventory, sales, and supplier data in plain language.
Use your inventory reporting. The FBA inventory and inventory ledger reports show when a SKU's available quantity hit zero, and the listing and account health tools flag stranded or suppressed inventory that was live but unsellable. Count both: true zero-stock days and days the listing existed but could not be bought.
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Stockout days are the days a SKU was not available to buy, whether because you were fully out of inventory or because the listing was live but unsellable. The short version: they are easy to overlook, but if you leave them in your velocity math they understate how fast the SKU sells and make you reorder late, and they also drag your IPI. Below is what counts as a stockout day, why they distort your numbers, how to find them, and how to handle them.
A stockout day is any day the customer could not buy, which is broader than just being out of inventory:
True zero stock. Available quantity hit zero and the listing showed unavailable.
Stranded or unfulfillable inventory. You had units, but they could not sell (a delisted ASIN, a fee or compliance hold), so from the customer's side you were out.
A suppressed listing. The listing was pulled from search or the buy box for a content or policy issue, so demand could not convert.
All three generate zero sales for a reason other than zero demand, which is exactly what makes them dangerous to treat as normal data.
The most common damage is to your sales velocity, and through it your reorder timing:
If you average units sold over a period that includes stockout days, the zeros pull the average down.
That understated velocity makes your days of supply read longer than it truly is.
So your reorder date lands too late, and you risk running out again, generating more stockout days in a self-feeding loop.
The fix is to exclude stockout days from the denominator: divide units sold by the days the SKU was actually available, not the total calendar days. Put numbers on it:
Calculation
Days
Units sold
Velocity
Including 6 stockout days
30
150
150 / 30 = 5.0/day
Excluding the 6 stockout days
24
150
150 / 24 = 6.3/day
Leaving the stockout days in understates velocity by more than 25 percent (5.0 vs 6.3), which makes your days of supply read long and your reorder land late. Excluding them gives you the real in-stock velocity your reorder math needs.
Beyond velocity, stockout days feed a metric Amazon watches directly. Your in-stock rate, the share of time your SKUs were available, is a component of the IPI. Frequent or long stockout days lower it, which drags the score that governs your storage limits. Amazon has used an IPI threshold (historically around 400) below which it restricts your restock quantities, so a falling in-stock rate can push you toward real restock limits. That threshold has changed over time, so check your own account, but the direction holds: a stockout costs you twice through this channel, the lost sales now and a weaker IPI that can restrict your restocking later.
Inventory and ledger reports. The FBA inventory and inventory ledger reports show when available quantity hit zero for each SKU.
Stranded inventory reports. These flag units that were present but unsellable, the hidden stockout days that never show as "out of stock."
Listing and account health. These surface suppressed or delisted periods when demand could not convert.
Count all of them. The obvious zero-stock days are only part of the total; the stranded and suppressed days are the ones sellers routinely miss.
One honest caveat on the hidden days: the native stranded-inventory and listing tools show current status, not history, so you cannot pull "ASIN X was stranded from the 3rd to the 9th" after the fact. Historical true-zero-stock days you can reconstruct from the inventory ledger, but for stranded and suppressed periods the practical fix is to snapshot your stranded-inventory report on a weekly cadence so you build the record over time, or use a third-party tool that logs availability. Do not assume Seller Central will hand you last quarter's stranded days; it will not.
Zero stockout days is the ideal but rarely realistic; the practical goal is a high in-stock rate:
Aim high on in-stock rate. As an operator rule of thumb, keeping your important SKUs available well above 90 percent of the time is a reasonable target, and Amazon's IPI rewards a strong in-stock rate.1 For your best sellers, treat any stockout as a problem to prevent, not absorb.
Weight by SKU value. A stockout day on a top revenue SKU costs far more than one on a long-tail item. Spend your prevention effort where the lost sales and rank damage are largest.
Watch the trend, not just the level. A rising stockout-day count across the catalog is an early signal that your reordering or safety stock is too lean, well before it shows up as a lower IPI.
The number that matters is your own trend on the SKUs that carry the business, not a universal target.
Stockout days are the days a SKU could not be bought, from true zero stock to stranded and suppressed listings. Left unhandled, they understate your velocity, push your reorders late, and drag your IPI. Find them in your inventory reports, exclude them from your velocity denominator, and use them to size lost sales. For the wider defense, see Amazon FBA stockout prevention.
The 90-percent-plus in-stock target is an operator rule of thumb, not a published Amazon threshold; Amazon rewards a strong in-stock rate through IPI but does not publish a single required figure. Set your own target from the value of the SKU and track the trend. ↩