Available FBA units you can sell today.
A 30-day average is a good default.
Shown separately, since inbound stock isn't sellable yet.
From placing the purchase order to units available at Amazon.
The formula
Days of inventory = sellable units ÷ average daily unit sales.
- Count units that can realistically satisfy demand today. Keep inbound stock separate, since it is not sellable yet.
- A 30-day sales window is a practical default. A 7-day window reacts faster but is noisier, and a 90-day window hides recent changes.
- Don't let stockout days drag down your average. Lost availability depresses observed sales, so the real demand is higher.
Frequently asked questions
How do I calculate days of inventory on Amazon?
Days of inventory equals sellable units divided by average daily unit sales. Three hundred sellable units selling 10 a day is about 30 days of cover, and the same 300 units at 25 a day is only 12.
Which sales window should I use for daily sales?
A 30-day window is the practical default. Compare it with the 7-day figure: if 7-day sales run well above 30-day, demand may be accelerating, and well below means the product may be cooling off.
Why does lead time matter for days of inventory?
Forty days of cover is not comfortable if replenishment takes 70 days from purchase order to available stock. In that case the cover is already shorter than the lead time, so you would run out before a new order could arrive.
Go deeper
- Days of Inventory for Amazon Sellers Explained
Stock level shows how many units you have, but days of inventory shows how much time you have left. Here is the formula and the right sales window to use.
- Amazon Sales Velocity: A Formula for Inventory Decisions
Sales velocity equals units sold divided by days in the period, though the window you pick and how you handle stockout days can change the number substantially.
- The Amazon Stockout Alert That Actually Matters
By the time a listing hits zero inventory, the damage to rank and Buy Box is already done. Days-of-stock forecasting catches the problem much earlier.
- Amazon FBA Inventory Statuses Explained
Amazon can hold 500 of your units while only 320 are available to sell. Available, reserved, inbound, unfulfillable, and stranded each mean something different.
EcomSanity calculates days of stock per ASIN from your real sales velocity and flags listings approaching a low-stock threshold.
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