Why a Fixed Low-Stock Number Lies to You: Days of Cover Explained
A fixed low-stock threshold treats a slow mover and a bestseller the same. Days of cover scales with velocity. Why it is the better alert to set.
Most low-stock alerts ask you for a number. “Tell me when this product drops below 20 units.” It feels precise and sensible. It is actually one of the least useful settings in inventory management, because 20 units means something completely different from one product to the next.
The problem with a fixed unit threshold
Picture two products, both set to alert at 20 units.
The first sells one unit a day. At 20 units, you have 20 days of runway before you run out. Plenty of time. The second sells ten units a day. At 20 units, you have two days. You are about to stock out and the alert has barely given you any warning.
Same threshold, completely different urgency. A fixed unit count is blind to how fast a product moves, which is the one thing that decides whether 20 units is comfortable or a crisis. And because it is blind to velocity, you have to set it individually for every product, and reset it every time demand shifts with a promotion or a season. Those numbers rot, and a catalogue full of stale thresholds is worse than none, because you trust them.
What days of cover does instead
Days of cover measures stock in time rather than units:
Days of cover = current units / current daily velocity
It answers “how many days until I run out at the current pace.” A low-stock alert built on it says “tell me when a product has fewer than X days left,” not fewer than X units.
The difference is that one setting now works for your whole catalogue. Set it to, say, ten days of cover, and the product selling one a day triggers at ten units while the product selling ten a day triggers at a hundred. Each is judged against its own speed, automatically. The same single rule fits a piano shop and a t-shirt shop alike.
Why this is the right language for an alert
The entire purpose of a low-stock alert is to tell you “you have just enough time to reorder before you run out.” That is a question about time and lead time, not a raw count.
This is where days of cover really earns its place. Set the threshold near your supplier lead time plus a small buffer, and the alert fires at exactly the last responsible moment to place an order. “Warn me when cover drops below my lead time” is a far more useful instruction than “warn me at 20 units,” because it is tied to the thing that actually matters: whether you can still reorder in time.
The edge case: products with no history
Days of cover needs velocity, and a brand-new product with no sales yet has none, so the calculation is undefined. The fix is a units floor as a backstop, a minimum count that still triggers an alert until the product has built up enough sales history to compute cover properly. Without that, new products would slip through unwatched.
How Stockful does it
Stockful’s low-stock threshold is days of cover, not a fixed count. You set the number of days, and it alerts when a SKU will run out in fewer than that at its current velocity, scaling per SKU automatically. A units floor covers products with no sales history yet, and when this replaced the old fixed-count threshold, existing settings were converted from each shop’s historical velocity, so nobody had to redo their thresholds by hand. Pair it with Slack alerts and the right products surface at the right time.
A fixed number tells you how much stock you have. Days of cover tells you how much time you have, and time is the only thing that decides whether you reorder today.
Get started free at stockful.app. Stockful’s low-stock alerts use days of cover, so one setting fits your whole catalogue.
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