A stockout is having no sellable inventory of a product when demand for it exists. The visible cost is the lost sale. The larger and less visible costs are wasted advertising spend pointing at an unbuyable page, support time answering when it will return, and customers who buy the same item somewhere else and stay there.
In more detail
Stockouts get treated as bad luck, which is why they repeat. Most of them are the predictable consequence of a threshold set from a quoted lead time, an average that no longer reflects demand, or a promotion nobody told the buyer about.
The three are worth separating because they have different fixes. A threshold problem is arithmetic. A demand problem is a review cadence. A promotion problem is a conversation between marketing and whoever buys stock, and it is usually the cheapest of the three to solve.
Not every stockout is worth preventing. Holding enough safety stock to never run out of a slow, low-margin line costs more than the sales it protects. The goal is to choose which stockouts you accept rather than discovering them.
How it is calculated
Stockout rate = product-days with zero sellable stock ÷ total product-days
Worked example
Across 30 days you track 40 products, giving 1,200 product-days. Six products were out of stock for a combined 48 days. That is a 4 per cent stockout rate.
The percentage on its own is close to meaningless. If those 48 days were on your two best-selling lines during a campaign, it is a serious month. If they were on six slow lines you were deliberately running down, it is a good one.
The figures above are invented to make the arithmetic legible. They are not a benchmark and should not be cited as one.
Where people get it wrong
- Counting only the lost sale. Ad spend continuing against a sold-out page is often the larger number, and it accrues until somebody notices.
- Treating every stockout as equally bad. Rank by margin and by whether the product was being promoted.
- Fixing the symptom by over-ordering everything. That converts a stockout problem into a dead-stock problem, which is worse because it is slower to notice.
- Not recording the cause. Without it, every stockout looks like bad luck and the same one recurs.
Common questions
How do you calculate stockout rate?
Count the product-days where a line had zero sellable stock, divided by total product-days in the period. Weighting by margin or by whether the line was promoted tells you far more than the raw percentage.
What does a stockout actually cost?
The lost margin, plus any advertising spend still pointing at the page, plus support time, plus the share of customers who buy elsewhere and do not return. Only the first is easy to measure, which is why the total is usually underestimated.
Should we aim for zero stockouts?
No. Never running out of a slow, low-margin line costs more in held stock than the sales it protects. Decide which stockouts are acceptable rather than treating all of them as failures.