Stock cover days: what the number means and how to read it

Stock cover days, also called days of cover, is the number of days your current stock will last if sales carry on at their current rate. Take the units you have on hand and divide by your average units sold per day. Twelve units and two sales a day is six days of cover.

The word cover is doing the work. It is not a measure of how much stock you own, and it is not a health score. It is a countdown. It answers one question: how long until this SKU hits zero if nothing changes and nothing arrives.

That makes it the only inventory number you can compare directly against a date. Units cannot be compared to a supplier's timeline. Days can.

A SKU is late to order the moment its days of cover drop below its lead time. That is the whole decision, and it is why cover days matter more than a stock figure on a shelf label.

Say your candle supplier takes 21 days from purchase order to goods on the shelf. A SKU with 30 days of cover has nine days of slack: you can place the order next week and still have stock when the pallet lands. A SKU with 14 days of cover is already a week past the point where a reorder could have arrived in time, and you are now choosing between a gap on the product page and paying for faster freight.

So the useful reading is always a subtraction. Days of cover minus lead time is the slack you have left, in days. When that number goes negative, the order is overdue, whatever the stock column says.

Cover days inherit every weakness of the sales rate underneath them, so three things are worth checking before you trust a figure.

• The window. A 7-day window after a busy weekend makes cover look alarmingly short. A 30-day window across a quiet spell makes it look comfortable. Same stock, different number.
• Burst sellers. Items that sell in bursts rather than evenly can run out well before an average-based cover figure expires. That gap is what safety stock exists to absorb.
• New or thin history. A SKU with a few days of sales has a sales rate, not a trend. Treat its cover days as a first guess and revisit it.

Cover days also say nothing about whether the stock is worth holding. Three hundred days of cover is not safety, it is cash sitting on a shelf.

Stock cover days sits in the middle of a small set of numbers that feed each other.

• Days of inventory on hand is the same arithmetic stated from the accounting side, with the formula set out in full on its own page.
• Sales velocity is the units-per-day figure in the denominator, and the window you choose for it changes every cover figure in your catalogue.
• Lead time is the number you compare cover against, and the only one Shopify cannot tell you.
• Reorder point turns the comparison into a stock level, so you get a threshold instead of a daily mental subtraction.
• Safety stock is the cushion you add for the bursts an average hides.
• Stockout is what happens when cover runs out before the delivery arrives.