Free tool
Days of supply is a ratio. Q4 attacks the denominator.
Amazon checks two historical days-of-supply numbers, and charges the low-inventory-level fee only when both fall under 28 days. Enter your position, get both metrics and the verdict — then stress it against peak velocity to see which ASINs cross the line without you doing anything wrong.
The ASIN's position
Both figures come from your own history. Pull average daily sellable units on hand and average daily units shipped for each lookback from the FBA inventory and shipped-units reports.
Long-term — 90-day lookback
Short-term — 30-day lookback
Peak stress test
The multiple is your own peak-to-baseline ratio. If you do not have one, last year's peak week divided by last September's weekly average is a defensible starting point.
How the fee is assessed
Historical days of supply = average daily units on hand / average daily units shipped, computed separately over 90 and 30 days and refreshed weekly. The fee applies only whenboth figures sit below 28 days. Per-unit rates vary by size tier and by how far below the threshold you sit — read the current rate for your ASIN in Seller Central's Fee Preview rather than assuming an average.
The read
Status today
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- 90-day DoS
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- 30-day DoS
Headroom
At peak velocity
Operator's verdict
Threshold used: 28 days on both metrics. Nothing you enter leaves your browser.
How does Amazon actually calculate historical days of supply?
Average daily inventory units divided by average daily units shipped, computed over two separate lookbacks — 90 days (long-term) and 30 days (short-term). It is recalculated weekly, typically Sunday night into Monday. It is a backward-looking ratio, not a forecast, which is why a demand spike shows up in it only after the fact.
Do I need both metrics above 28 days to avoid the fee?
No — you need one. The low-inventory-level fee applies only when both the 90-day and the 30-day historical days of supply sit below 28 days. Clearing the threshold on either metric waives it. That asymmetry is why a long, slow history can shield you through the first weeks of a ramp while the 30-day number is already collapsing.
Why does peak season push ASINs into the fee that were fine all year?
Because days of supply is a ratio and Q4 attacks the denominator. Ship three times as many units per day and the same inventory position becomes a third of the days of cover, without you selling out or making a single planning error. An ASIN sitting at 40 days in September is at 13 days of cover at 3x velocity — below the line, on inventory that never moved.
Related reading: Low-inventory-level fee ·Sell-through rate ·AWD ·Size tier checker · or have an operator run the catalog.
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