Quick answer: Amazon's low-inventory-level fee applies only when both 30-day and 90-day historical days of supply are below 28 days for an eligible product, a backward-looking metric that often doesn't match what a simple stock-divided-by-velocity spreadsheet shows.
Amazon has managed to create a situation where sellers can be charged for holding too much FBA stock and, in some cases, for holding too little.
That tension is why the low-inventory-level fee causes so much frustration. The seller is asked to keep enough inventory in Amazon's network to support efficient delivery, while also avoiding storage costs and aged-inventory surcharges. The answer isn't "send more of everything." It's understanding how Amazon measures supply and which products actually face the fee.
What the fee is trying to measure
In the US, Amazon applies the low-inventory-level fee to eligible standard-size FBA products when inventory has remained low relative to historical demand. Amazon calls the metric historical days of supply. It looks at inventory levels relative to sales and evaluates both a short-term and long-term period. As of 2026, the fee applies only when both are below 28 days: short-term historical days of supply based on the last 30 days, and long-term historical days of supply based on the last 90 days. If either measure is at or above 28 days, the fee doesn't apply for that product during the relevant calculation. That "both" matters. Sellers often see one low number and assume the fee is unavoidable.
Historical days of supply isn't today's stock divided by today's velocity
This is where many spreadsheet comparisons go wrong. A seller looks at 60 units in stock, sees the product sells 2 a day, and concludes there are 30 days of supply. Amazon's historical metric can show something different because it's based on inventory history, not only the current snapshot. The metric is backward-looking: it asks whether the product has consistently had enough stock relative to demand during the measurement windows. That means a large inbound shipment arriving today may not instantly repair the long-term measure. It can improve the short-term position over time, but the historical record still exists.
Why sellers struggle to reproduce the number
Seller forum discussions repeatedly show the same complaint: "my calculation says 50 days, Amazon says 22." The disagreement can come from several places: Amazon may use average inventory over the period rather than today's on-hand units, the seller may include inbound stock Amazon doesn't yet treat as available, sales timing and inventory snapshots may not align with the seller's spreadsheet, and days when stock was low can continue to affect the historical measure even after replenishment. Treat your own days-of-stock calculation as an operating forecast, covered in days of inventory for Amazon sellers. Treat Amazon's historical days of supply as the fee metric. They answer related but different questions.
Which products are commonly excluded
Amazon's help documentation lists exemptions that can change over time. New professional sellers and newly enrolled FBA products may receive temporary relief. Products managed through certain Amazon supply programs may be treated differently, and products selling below a specified unit threshold can fall outside parts of the policy depending on current rules. Don't rely on an old blog's exemption list; check the current marketplace help page and the FBA Inventory view for the product itself.
Running out of stock is not a smart fee strategy
Amazon has stated in seller-forum guidance that the low-inventory fee isn't charged while a product is out of stock, which has led some sellers to joke that zero inventory is cheaper than low inventory. Technically true: a fee on fulfilled units can't apply when no units are being fulfilled. Commercially, deliberate stockouts can cost much more through lost sales, weaker ranking, disrupted advertising, and slower recovery, a tradeoff covered in the Amazon stockout recovery plan. Avoiding a small per-unit fee by losing the sale is usually not a victory.
The practical target isn't exactly 28 days
Aiming for 28.1 days leaves no room for receiving delays, sudden demand, or calculation differences. Amazon's own event-readiness guidance has recommended entering major deal periods with roughly four to six weeks of inventory, a useful operational range for stable, replenishable products, though not universal. A reasonable buffer should reflect demand variability, supplier reliability, inbound transit time, Amazon receiving speed, product size and storage cost, cash availability, and seasonality. Bulky, slow products may not justify six weeks inside FBA. Fast, compact products with reliable demand may need more.
A worked example
An ASIN sells 300 units in a typical 30-day month. Current FBA stock is 180 units, so the simple forward-looking estimate is 18 days. The seller has 240 units in transit, but Amazon hasn't received them yet. Amazon's historical days of supply may also remain below 28 because the product spent much of the previous month understocked. The seller can't change the past but can change the next few weeks: expedite or redirect replenishment where practical, reduce wasteful advertising that accelerates low-margin demand, review price rather than blindly cutting it, and keep the product above the threshold long enough to repair the short-term history. The key is consistency, not one dramatic shipment.
The fee creates a balancing problem with aged stock
At one end, low stock can trigger a per-unit fee. At the other, excess stock creates monthly storage and aged-inventory surcharges. The best inventory position is a moving band, not a fixed number: above lead time and low-inventory risk, below the point where cash and storage costs become excessive. That's why sellers need three separate views: forward days of stock based on current sales velocity, Amazon's historical days of supply for fee exposure, and inventory age for overstock exposure. Combining them into one label called "inventory health" hides too much.
EcomSanity calculates forward-looking days of inventory from actual sales velocity and flags stockout risk across the catalog. That doesn't replace Amazon's historical fee metric, but it helps prevent the repeated low-stock pattern that creates exposure in the first place.
Frequently asked questions
When does Amazon charge the low-inventory-level fee?
When both short-term (30-day) and long-term (90-day) historical days of supply are below 28 days for an eligible standard-size FBA product. If either measure is at or above 28 days, the fee doesn't apply.
Why does my own days-of-stock calculation not match Amazon's historical days of supply?
Amazon's metric is backward-looking, based on inventory history over the measurement window, not today's on-hand units divided by today's velocity. A large inbound shipment arriving today doesn't instantly repair the long-term historical measure.
Does the fee apply while a product is out of stock?
No. A fee on fulfilled units can't apply when no units are being fulfilled. But a deliberate stockout to avoid a small fee usually costs far more in lost sales, weaker ranking, and slower recovery than the fee itself would have.