Days of cover: formula, meaning and common mistakes

Days of cover answers one question: if nothing new arrives, how long until this SKU runs out? Teams also call it days on hand or days of inventory, and Amazon calls it days of supply. Finance uses similar names, such as days in inventory or days sales of inventory, for DIO, which is measured in inventory value rather than units, so check which one a report means.

How do you calculate days of cover?

Days of cover = sellable units ÷ average daily unit sales

A SKU with 600 sellable units at FBA that sold 20 a day over the last 30 days has 30 days of cover.

Two inputs decide whether it is right:

• Which units. Amazon's FBA Inventory API splits stock into fulfillable, inbound, reserved, unfulfillable and researching. Fulfillable units can ship now. Reserved units held for customer orders are already committed, so leave them out. Amazon's historical days of supply counts units moving between fulfillment centers or in FC processing as sellable, so include them. Inbound stock counts once it is received.
• Which rate. A trailing average looks back; a forecast looks forward. Before a deal, a season or a bigger ad budget, the trailing rate overstates cover.

How is it different from days of supply and DIO?

Term	How it is calculated	Where it is used
Days of cover	Sellable units ÷ average daily unit sales	Replenishment, per SKU and location
Weeks of cover	Sellable units ÷ average weekly unit sales	Same idea in weeks; Amazon reports 30-day and 90-day versions
Days of supply (Amazon)	How long stock at Amazon will last; the Restock total includes open shipments	Seller Central Restock and inventory health reports
Historical days of supply (Amazon)	Average daily sellable FBA units ÷ average daily shipped units, over 30 and 90 days, per FNSKU	Amazon's low-inventory-level fee
Days inventory outstanding	Inventory value ÷ cost of goods sold × days in the period	Finance, whole company

Amazon's Manage Inventory Health report carries its days of supply, short- and long-term historical days of supply and weeks of cover per SKU, and flags whether the low-inventory-level fee applied that week or the product is exempt.

Why does days of cover matter?

It turns stock into a date. Compare it with your full lead time, from purchase order to sellable at FBA. When cover drops to lead time plus safety stock, order now: that crossing point is your reorder point. If cover is already below lead time and nothing is on order, an order placed today lands after you run out. Amazon's own guide writes the reorder point as average daily sales × lead time + buffer stock.

Amazon's inventory rules push on both ends:

• Too little. In the US, the low-inventory-level fee applies to eligible products when both the 30-day and 90-day historical days of supply are under 28 days. Exemptions include new Professional sellers for 365 days after their first inventory is received, new-to-FBA parent products enrolled in FBA New Selection for 180 days after their first inventory is received, SKUs with 70% or more of units auto-replenished through AWD over the prior 90 days, products that sold fewer than 20 units in the past 7 days, and Grocery.
• Too much. More than 90 days of supply and units aged over 90 days are among Amazon's signs of excess inventory, and it suggests about two months of supply. Excess ties up cash, adds monthly storage fees and, from 181 days of age, the aged inventory surcharge.

Cover should also steer ads. A SKU with 10 days of cover and a 45-day lead time should not get more budget, however strong its ROAS. See inventory-aware ad spend.

What mistakes make days of cover misleading?

• Counting inbound units or units reserved for customer orders as sellable. Cover looks fine until orders stop.
• Averaging over stockout days. Days at zero stock sell zero and make cover look longer. Leave them out.
• Ignoring other channels. If FBA stock also fills Shopify orders through multi-channel fulfillment, count that demand too. Amazon's historical days of supply already counts those shipments.
• Blending locations. Ninety days at a 3PL and five at FBA is still an Amazon stockout next week.
• One threshold for every SKU. A 10-day domestic reorder and a 70-day ocean lead time need different thresholds.

How does Growth OS track days of cover?

Ecomsellertool Growth OS is built to compute cover per SKU, channel and stock location on your own accounts:

1. Stock from the SP-API and your warehouse or 3PL, with inbound tracked by expected receive date.
2. Demand per channel, excluding stockout days, with planned promotions added to the forecast when you enter them.
3. Lead time per SKU, taken from your actual PO and receiving dates rather than a supplier's quote.
4. Action. When projected cover falls to lead time plus safety stock, the owner gets an alert with a draft PO or transfer, and ad rules can stop scaling that SKU.

You own the custom modules we build and your data; the Growth OS base is licensed to you.

Where should you start?

If days of cover lives in a weekly spreadsheet, or nobody can say which SKUs run out before the next shipment lands, start with the free Quick Scan in the Ops Gap Diagnostic. It looks for where stock, lead times and ads disconnect. See also replenishment or Growth OS.