GLOSSARY · UPDATED OCT 20, 2026
What is days of supply on Amazon?
Days of supply is how many days your stock will last at the current rate of sales: units available divided by average daily units sold. On Amazon it also decides a fee. Amazon charges the low-inventory-level fee only when both your 30-day and your 90-day historical days of supply are below 28 days, so the number has a floor as well as a ceiling.
The days of supply formula
- Days of supply
= units available ÷ average daily units sold
- Days of supply including inbound
= (units available + units inbound) ÷ average daily units sold
- Average daily units sold
= units sold in the window ÷ days in stock in the window
Calculate it per SKU and per location. Use a 30 to 90 day window and leave out days you were out of stock, or the average falls and days of supply looks healthier than it is.
Worked example
Illustrative · sample figures. One SKU sells 15 units a day. Lead time from purchase order to sellable stock at FBA is 62 days.
| Location | Units | Days of supply |
|---|---|---|
| FBA | 300 | 20 |
| Your 3PL | 900 | 60 |
| Total | 1,200 | 80 |
In total the SKU looks safe: 80 days of stock against a 62-day lead time. At Amazon it is not. Twenty days at FBA is under the 28-day line, and replenishment from the 3PL takes time to be received. If Amazon's 30-day and 90-day historical measures are both below 28 when units ship, the low-inventory-level fee applies to those units. If the 90-day measure is still at, say, 35 because stock was higher last quarter, it does not. The fix is a transfer from the 3PL now, sized so FBA holds more than 28 days after receiving.
How Amazon measures days of supply for the low-inventory-level fee
Amazon calculates historical days of supply itself, over a short-term window (the last 30 days) and a long-term window (the last 90 days), and assesses the fee only if both are below 28 days, according to an Amazon staff explanation in Seller Forums. Either one above 28 avoids it. Amazon also applies exemptions. In that explanation they covered new Professional sellers for their first 365 days and new-to-FBA products enrolled in FBA New Selection for 180 days. Seller Labs reports that since Jan 15, 2026 the fee also covers small and large bulky items and excludes oversize, apparel and hazmat (vendor, updated Aug 3, 2026). Check the current rules and per-unit amounts in Seller Central before you plan around them, because Amazon updates both.
How days of supply works on Walmart and Shopify
Walmart.
WFS charges storage by the cubic foot and adds long-term storage on inventory older than a year: $2.25 per cubic foot a month at 366 to 450 days and $7.50 after 450 days, since Jun 30, 2026 (Walmart, updated May 27, 2026). Days of supply far above 365 on a WFS SKU is a direct cost. The WFS storage fee calculator shows when removal costs less.
Shopify.
There is no platform fee tied to days of supply, but your 3PL charges storage, and cash sits in the boxes. Days of supply on Shopify stock is mainly a cash and reorder number.
Across channels.
If the same units serve Amazon, Walmart and Shopify, compute days of supply on combined sales for reorder decisions and per location for transfers.
Common mistakes
-
One total for all locations.
Ninety days in total can hide ten at FBA.
-
Counting stockout days in the average.
Days with no stock pull the average down.
-
Ignoring inbound.
Units on the water are not available, but they are coming. Track both numbers.
-
Running FBA lean to save storage.
Below 28 days, the low-inventory-level fee can cost more than the storage you saved.
-
Running it fat for Q4.
Storage costs more from October to December, and slow stock heads toward aged inventory surcharges.
What is a good days of supply for Amazon FBA?
For steady sellers, enough to stay above 28 days of historical supply at FBA, so the low-inventory-level fee does not apply, while keeping total stock within your lead time plus safety stock. For many SKUs that means 30 to 60 days at FBA with the rest at a 3PL, depending on receiving times and storage cost.
Is days of supply the same as days of inventory?
They measure the same idea from different directions. Days of supply looks forward: how long current stock lasts at current sales. Days of inventory, in the cash conversion cycle, looks back: average inventory ÷ cost of goods sold × 365. Both rise when stock outpaces sales.
Why did I pay the low-inventory-level fee with 3PL stock on hand?
Because Amazon measures days of supply at FBA, not across your network. Units at your 3PL or in transit do not count until Amazon receives them. Plan transfers so FBA stays above 28 days after receiving time.
Does the low-inventory-level fee apply to every product?
No. Amazon applies exemptions and size-tier rules. A vendor summary says oversize, apparel and hazmat products are excluded and new FBA products are waived for a period (Seller Labs, Aug 3, 2026). Confirm in Seller Central for each SKU.
How often should I check days of supply?
Weekly for most SKUs and daily for top sellers in peak season. A weekly check catches an FBA level drifting toward 28 days in time to ship from the 3PL.
Can you watch this for us?
Yes. Stock Flow builds days of supply and reorder alerts per SKU and location in your own accounts. See pricing for the fee.
Sources3 sourcesLast checked Oct 20, 2026
- 01sellercentral.amazon.comAmazon staff, "Low-inventory-level fee further explained": short-term 30-day and long-term 90-day historical days of supply; fee only if both are below 28 days; exemptions at the time; about 2023; checked Oct 20, 2026Primary
- 02sellerlabs.comfee extended to small and large bulky from Jan 15, 2026; oversize, apparel and hazmat excluded; new-product waiver; updated Aug 3, 2026; vendor; checked Oct 20, 2026Vendor
- 03marketplacelearn.walmart.comstorage $0.75 per cubic foot; long-term from Jun 30, 2026; updated May 27, 2026; checked Oct 20, 2026Primary
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