Amazon FBA Days of Supply & the 2026 Low-Inventory Fee

By Bernardo Campelo, Forbes Business Council E-Commerce Leader, Amazon SPN Certified provider, Amazon SP-API authorized partner, and Founder of PrepVia.

A seller messaged me last quarter in a full-blown panic. Seller Central said his hero SKU had four days of cover left, his Restock report was screaming red, and he was about to wire his factory for an emergency air-freight reorder that would have cost him four figures in expedited shipping. There was just one problem. Six hundred units of that exact SKU were already received and prepped, sitting on a shelf at the prep center, waiting for a draft shipment. Amazon had no idea they existed. His “four days of cover” was a lie, and it almost cost him a fortune.

That is the whole problem with days of cover in FBA. The number sellers panic over is built on what Amazon can see, and Amazon cannot see your whole pipeline. So let me give you the real formula, the fee that now punishes you for getting it wrong, and the buckets Amazon leaves out.

The 60-second version

“Days of cover” and “days of supply” answer the same question, how long your inventory lasts at your sales rate, but Amazon runs two different metrics: Estimated Days of Cover in the Restock report, which counts inbound, and historical days of supply, which does not. The fee runs on historical days of supply, measured per FNSKU and updated weekly, and it is the number behind the Low Inventory Level Fee. Get it wrong on the low side and Amazon charges you a per-unit fee; get it wrong on the high side and your cash is trapped in overstock. The catch is that the fee metric only counts the units Amazon is holding, not what is in transit or sitting at your prep center, so the figure that drives your reorder decisions is structurally incomplete.

What the Low Inventory Level Fee actually is

This is the fee that made days of supply everyone’s problem. Amazon charges a per-unit Low Inventory Level Fee on standard-size, Small Bulky and Large Bulky items, with Grocery exempt, when both your 90-day and 30-day historical days of supply sit below 28 days, on the logic that thin, fast-churning inventory is expensive for it to fulfill. Reported amounts run roughly $0.32 to $2.09 per unit depending on the shortfall, confirm the current figures and thresholds in Seller Central. The cruel part is that the same thin inventory that costs you the sale also adds the fee to every unit you do ship. Forecasting days of supply correctly is how you stop paying Amazon for running lean.

How Amazon calculates days of supply

The formula is average daily inventory units divided by average daily shipped units, run over the last 90 days and the last 30 days, not today’s stock over today’s sales. Amazon charges only when both windows sit below 28 days, so a single bad month does not sink you, and pushing the 30-day window back above 28 days is enough to clear the fee. The mistake almost everyone makes is the input, not the math, they count only the units Amazon can already see.

Count the whole pipeline

The fix is to forecast against every unit you actually control, not just the slice Amazon holds. PrepVia’s forecasting computes days of cover across all three buckets, FBA-sellable, Amazon-inbound, and on hand at the prep center, so you reorder against your true position instead of the partial one Seller Central shows. That is the difference between the seller who wires for panic air freight and the one who already had six hundred units on a shelf. Restock timing also has to account for prep and freight lead time, see turnaround time, and over-buying to feel safe just traps cash, which ties back to FBA cash flow. It is one of the thirteen tools every Amazon seller should expect from a prep center.

Frequently Asked Questions

What is days of supply (days of cover) for Amazon FBA?

It is how many days your current inventory will last at your current sales rate. For the fee, Amazon calls it historical days of supply and calculates it as average daily inventory units divided by average daily shipped units, over the last 90 days and the last 30 days. It drives both your reorder timing and Amazon’s Low Inventory Level Fee.

What is the Amazon Low Inventory Level Fee and how do I avoid it?

It is a per-unit fee of $0.32 to $2.09, charged on standard-size, Small Bulky and Large Bulky items, with Grocery exempt, when both your 90-day and 30-day historical days of supply fall below 28 days. Confirm current values in Seller Central. You avoid it by forecasting across your whole pipeline, FBA-sellable plus in-transit plus prep-center stock, and restocking until the 30-day window clears 28 days. Amazon also exempts new Professional sellers for 365 days after their first inventory is received, new-to-FBA parent products for 180 days if they are enrolled in FBA New Selection, SKUs with 70% or more of their inventory auto-replenished from AWD over the prior 90 days, Grocery, and products that sold fewer than 20 units in the past 7 days.

Why is Amazon’s days-of-supply number wrong?

It is not wrong so much as incomplete. Amazon’s fee metric counts only the sellable units inside its own network, so units still inbound and units sitting at your prep center do not show up, even though the Restock report’s days of cover does count inbound. That makes the number understate your true position and can push you into an unnecessary, expensive emergency reorder.

How many days of inventory should I keep in Amazon FBA?

Enough to cover your reorder lead time plus a safety buffer, without drifting so high that cash is trapped in overstock or you risk aged-inventory costs. The right target depends on your sales velocity and supplier lead time, which is why forecasting against your full pipeline beats reacting to Amazon’s partial number. For the operational side, see how to reduce inventory cycle time.