Days of cover answers one question: at the rate we are selling, how long until this runs out? Ad accounts that never see that number optimise for the wrong win.
You can calculate a rough version as on-hand units divided by average daily units sold. Use a window that matches how jumpy your category is. Seven days if you move fast. Fourteen or thirty if you are steadier.
Why marketing teams skip it
It is not in Ads Manager. It is not in most creative briefs. It lives with ops or in a dusty sheet. So acquisition keeps scaling the hero that is about to vanish, then blames “creative fatigue” when reality was inventory.
How to use DOC without becoming a supply chain analyst
- Tag any SKU under your threshold before you write new ad copy for it.
- Refuse to build a big prospecting push on SKUs you cannot restock in time.
- When DOC is high and sell-through is dead, that is a different project: clearance, bundling, or stop buying attention for it.
Healthy ROAS on a product with three days of cover is not a win. It is a countdown.
Pair it with velocity, not vibes
Cover alone can lie after a one-off spike. Glance at velocity trend. If yesterday was a gift card spike and today is normal, do not rebuild the whole account.
For Shopify operators, the job is boring consistency: stock truth next to spend truth. That is the backbone of sensible store optimisation, not another vanity dashboard tile.
Make it a habit, not a project
Weekly is better than never. Daily is better if you scale hard. Put the number next to spend in whatever you already open in the morning. Consistency beats a perfect formula you abandon.
Also watch returns. High sell-through with high returns is not demand. It is a product or expectation problem wearing a sales costume.
Questions people actually ask
Is weeks of supply the same idea?
Same family. Pick one language so people stop converting units mid-meeting.
What window for daily sell rate?
Start with 14 days for most DTC. Shorten if promo-noisy; lengthen if volume is thin.
Worked example (simple numbers)
You have 140 units. Last 14 days sold 70 units. Daily rate is 5. Cover is 28 days. That is usually fine for modest prospecting.
Same 140 units, but last 7 days sold 70 because of a promo. Daily rate 10. Cover is 14 days. If the promo ends and velocity halves, cover doubles again — do not overreact on one noisy week without checking the promo calendar.
If cover is 28 days but 120 of those units are a colour nobody buys, your “cover” is a lie. Segment by the variants ads actually push.
Share the number in the creative brief
If designers and buyers never see cover, they will keep featuring what looks good, not what you can sell. A one-line stock note in the brief is unglamorous and extremely effective.
If you want stock, ads and the morning list in one place: that is what we built Ralph for — a growth operator for Shopify, with review before anything ships. Not magic. Fewer tabs. See Ralph · Docs