A stockout’s visible cost is the sales you did not make; its real cost adds what the gap does to your search standing, your metrics, and your reorder economics. Pricing it honestly matters because every prevention lever, safety stock, faster reorders, better forecasts, costs something visible, and unpriced stockouts always look cheaper than the cure.
The formula, line by line
Line 1: lost margin during the gap. Daily velocity x gap days x unit margin. A SKU selling 5 a day at $12 margin, out for 8 days: $480. Use the velocity BEFORE the stockout, days-of-cover data gives it to you, and be honest about gap length: order-to-shelf, not order-to-ship.
Line 2: substitution and channel effects (a credit, sometimes). Some buyers take your alternative SKU (recovered margin); some buy elsewhere and some of THOSE never return. For catalogs with real substitutes, credit line 1 by your substitution rate; for one-product businesses, add a customer-lifetime debit instead. Multichannel wrinkle: if only one channel stocked out (allocation error rather than true zero), the cost is a pool-management failure, cheaper to fix, embarrassing to repeat.
Line 3: marketplace momentum damage. Marketplace search rewards recent sales velocity; a stockout zeroes it. On return, the listing climbs back instead of resuming, days-to-weeks of depressed visibility that outlast the gap itself, the eBay mechanics, and on Amazon a stocked-out offer loses the flywheel exactly as long. Estimate: an extra 25-100 percent of line 1, worse in competitive categories. This line is why marketplace-heavy sellers weight prevention higher than store-first sellers.
Line 4: the scramble premium. Emergency reorders cost more: air freight over ocean, broken price breaks, expedited receiving. Whatever your last scramble cost above the calm version, that is the line.
Worked total: the $480 gap above + 50 percent momentum drag ($240) + a $150 expedite = $870 for one 8-day stockout on one mid-velocity SKU. Three of those a quarter quietly outspends most sellers’ entire tooling budget.
What the number changes
- Buffer sizing gets rational: holding 2 extra units of that SKU costs a few dollars a month against an $870 downside, the comparison prevention math wants.
- Reorder points move earlier for high-line-3 SKUs: marketplace flagships deserve triggers padded beyond the standard formula.
- A-items get watched: the ABC discipline exists because line 1 scales with velocity, an A-item stockout is the expensive kind by definition.
- Forecast investment justifies itself: stockout forecasts that buy you 10 extra days of warning convert scramble premiums into calm reorders.
Stockouts you choose versus stockouts that happen
Deliberate gaps exist, end-of-life SKUs, seasonal exits, capital triage, and they cost lines 1-3 too; choosing them with the number in view is strategy. The stockouts worth engineering away are the surprises: forecast misses, count drift, and sync failures that let a channel sell what the pool no longer had, the prevention stack.
Common questions
Is a stockout worse than overstocking?
They are the two tails of the same distribution: stockouts cost margin and momentum; overstock costs holding and obsolescence. The honest answer is per SKU, which is why cover TAILS, not averages, run the dashboard, the KPI view.
How do I measure lost sales I never saw?
Pre-gap velocity is the best proxy; marketplace traffic reports (sessions on a zero-stock listing) sharpen it. Precision matters less than not using zero.
Do stockouts hurt my own store’s SEO too?
Persistent out-of-stock pages erode organic performance over time, though gentler than marketplace rank. Keep the page, state the return date, offer the substitute.
What is the cheapest stockout insurance?
Accurate counts and early warning: record accuracy plus continuous cover forecasts prevent the surprise class outright; buffers then cover the residual forecast error on the SKUs that matter.
Price the gap, then prevent it
Low-stock alerts across every channel, prevention priced from $49/month with unlimited orders, with per-SKU velocity and cover forecasts on the plans above. See pricing.