Days of Supply
How many days your stock lasts at current sales velocity.
Definition
Days of supply is how many days your current on-hand inventory will last at your recent sales velocity. You calculate it by dividing units available by average units sold per day. If you have 600 units and sell 20 per day, you have 30 days of supply.
Why days of supply matters for an FBA seller
Unit counts alone do not tell you whether you are about to stock out. Five hundred units is comfortable for a slow SKU and dangerously thin for a fast one. Days of supply converts raw inventory into the only thing that matters: how much runway you have left.
It is the number that turns a sudden velocity spike into an early warning instead of a Monday-morning surprise. When days of supply drops below your replenishment cycle plus lead time, you are already late.
The traditional formula is backward-looking, and that is the wrong way to plan
The standard days of supply calculation divides current on-hand units by historical sales velocity. It is a snapshot of the past projected onto a flat line. That works fine for a product that sells the same number of units every week forever, and almost no Amazon SKU does. Seasonality, promotions, ranking shifts, competitor stockouts, and pricing changes all bend future demand away from the trailing average, often by a lot.
When you run a business on backward-looking days of supply, you are essentially betting that next month looks like last month. Sellers who plan that way are the ones who get caught in Q4 with comfortable-looking 45-day numbers in October and an out-of-stock SKU on Black Friday.
How Inventory Hero does this differently
Inventory Hero replaces the backward-looking formula with a forward projection. Instead of dividing on-hand units by what you sold last week, we forecast what your unit velocity will be over the coming weeks and months and then walk your inventory down against that forecast to find the actual day you will run out.
The forecast uses modern demand modeling on your real sales history, seasonality patterns at the SKU level, and configurable growth assumptions, then updates continuously as new orders come in. The number you see is days of runway against expected future demand, not against a frozen trailing average. That gives you a stockout date you can plan a PO around, instead of a number that flatters you right up until it betrays you.
How days of supply connects to your restock decisions
Days of supply is the trigger for a purchase order. The rule is simple: place the order before your remaining days of supply fall below your supplier lead time plus a safety buffer, or you will run out before the next shipment lands.
Because velocity changes with seasonality and promotions, days of supply has to be recalculated continuously, not once a month. Stale velocity is how sellers go from comfortable to out of stock in two weeks. Use a forward-looking velocity, not a flat trailing average, when demand is trending.
Related terms
See it applied in Inventory Hero
Inventory Hero turns these inputs into restock recommendations against your real Amazon SKUs.
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