Weeks of cover: what it means for inventory
Weeks of cover is the number of weeks your current stock will last at the rate you are selling it. It is the same idea as days of inventory remaining, expressed in the unit most merchants actually think in when they talk to a supplier.
What weeks of cover means
Weeks of cover is units on hand divided by units sold per week: the number of weeks your stock will last if demand holds steady. Four hundred units on hand against fifty units a week is eight weeks of cover.
The number is a ratio, not a count, and that is the point. Two hundred units sounds comfortable on a slow SKU and reckless on a fast one. Cover collapses both facts into one figure you can compare across a catalogue of fifty SKUs or two thousand.
Where the sales rate comes from
You pick a window of recent history and average it. A short window, say the last seven days, reacts fast and swings hard. A longer window, say thirty days, is steadier and slower to notice a shift. Neither is correct in the abstract: the right window depends on how volatile the SKU is and how long your supplier takes.
Reading it against lead time
Cover only becomes a decision when you put it next to your supplier's lead time. If a SKU has three weeks of cover and the factory needs four weeks to deliver, you are already late and the only question is how bad the gap gets. If it has eight weeks of cover against a three week lead time, you have roughly five weeks of slack before the order has to go out.
That subtraction is the whole job. Cover minus lead time is the runway you have left to act, and running out of a SKU that sells well is a real hit to the week, so most merchants keep a buffer on top rather than cutting it to zero.
A worked example across three SKUs
| SKU | Units on hand | Units sold per week | Weeks of cover | Supplier lead time | The call |
|---|---|---|---|---|---|
| Sandalwood candle | 420 | 60 | 7.0 | 3 weeks | Four weeks of slack, review next week |
| Beeswax taper pair | 84 | 42 | 2.0 | 3 weeks | Already inside lead time, order today |
| Linen apron | 150 | 10 | 15.0 | 2 weeks | Heavily covered, hold the cash |
Related terms
- Days of inventory remaining. The same calculation in days rather than weeks. Divide weeks of cover by seven, or multiply days by seven to go the other way. Dashboards tend to prefer days because the number is larger and the change is easier to see.
- Sales velocity. Units sold per day or per week, averaged over a chosen window. It is the denominator of cover, so every argument about cover is really an argument about which window to average.
- Lead time. How long your supplier takes between your order and stock on your shelf, including shipping and inbound handling. A merchant input, not something a sales history can tell you.
- Reorder point. The stock level at which you place the order, usually lead time demand plus a buffer. Cover tells you where you stand; the reorder point tells you where the line is.
- Safety stock. The buffer you hold above lead time demand to absorb a spike or a late shipment. It is why most merchants reorder at three or four weeks of cover rather than at exactly their lead time.
- Stockout. The state cover is meant to prevent: zero units on a SKU customers still want.
- Weeks of supply. Another name for the same ratio, more common in wholesale and retail buying than in ecommerce.
If your demand is not steady, the averaging window matters more than the formula does. Our guide on planning for a seasonal spike covers choosing a short velocity window before a peak.
Questions
How do you calculate weeks of cover?
Divide units on hand by average units sold per week. Sixty units sold a week against three hundred units on hand gives five weeks of cover. Choose the sales window deliberately, because the same SKU can read very differently on a seven day average than on a thirty day one.
Is weeks of cover the same as days of inventory remaining?
Yes, it is the same ratio in a different unit. Days of inventory remaining divided by seven is weeks of cover. Merchants often speak in weeks because supplier lead times are quoted in weeks, while software tends to display days.
How many weeks of cover should I hold?
Start from your supplier lead time and add a buffer for variability. A SKU with a two week lead time and steady sales needs far less cover than one shipped by sea from overseas with lumpy demand. There is no universal target, only a target relative to your lead time and your tolerance for running out.
What sales window should I average over?
Shorter windows follow recent demand closely and react to a spike quickly, at the cost of noise on low volume SKUs. Longer windows are steadier but slower to register a change in trend. If you are heading into a peak, a shorter window tells you the truth sooner.
Does weeks of cover include safety stock?
Not by itself. Cover measures all the stock you have, buffer included, so it will not tell you how much of that stock is your intended cushion. Set your reorder point above lead time demand if you want a buffer that is explicit rather than implied.
What happens to cover on a SKU with no recent sales?
The denominator is zero, so cover is undefined rather than infinite. Treat those SKUs as a separate list to review by hand, because the usual reorder maths cannot say anything useful about them until some sales history exists.