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” are the same thing: how many days your inventory will last at your current sales rate. Amazon uses days of supply, 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 Amazon’s number only counts the units it 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 items when your historical days of supply runs too low for too long, 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 a stockout can trigger the fee and cost you the sale at the same time. Forecasting days of supply correctly is how you stop paying Amazon for running lean.
How Amazon calculates days of supply
The core formula is simple: days of supply equals units on hand divided by average daily units sold. Amazon evaluates it over both a roughly 30-day and 90-day window, so one good week will not save you and one bad week will not sink you. The mistake almost everyone makes is the input, not the math — they count only the units Amazon can already see.
| Inventory bucket | Counted by Amazon? |
|---|---|
| FBA-sellable (at the fulfillment center) | Yes |
| Inbound to Amazon (in transit) | Partially |
| On hand at your prep center, received and prepped | No |
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, calculated as units on hand divided by average daily units sold. Amazon calls it days of supply and evaluates it over roughly 30-day and 90-day windows. 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, reported around $0.32 to $2.09, charged on standard-size items when your days of supply stays too low. Confirm current values in Seller Central. You avoid it by forecasting days of supply across your whole pipeline — FBA-sellable plus in-transit plus prep-center stock — and reordering before you run lean.
Why is Amazon’s days-of-supply number wrong?
It is not wrong so much as incomplete. Amazon only counts the units it is holding, so inventory in transit or sitting received at your prep center does not show up. 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.
See pipeline-aware forecasting in the PrepVia app →
