Ask a retail buyer what metric they look at first when they open their planning dashboard and most will say sell-through percentage. It's intuitive — a higher percentage means inventory is moving faster. But sell-through rate is a lagging indicator. By the time it tells you something is wrong, you've already missed the window to fix it.
Weeks of cover (WOC) is a leading indicator. It tells you, right now, how many weeks your current stock will last at the current rate of sale. That's the number that should be driving your reorder decisions.
If you have 300 units on hand and you're selling 30 units per week, you have 10 weeks of cover. Simple. The reason this metric is powerful is that it converts any inventory position — whether you have 50 units or 50,000 — into a single actionable number: time.
Time is the dimension that buyers actually work in. You don't care that you have 300 units. You care that 300 units runs out in 10 weeks, and your vendor lead time is 12 weeks.
The formula looks simple, but there are three common errors that produce a misleading number.
If you calculate average weekly units sold using 52 weeks of history, you're averaging your current demand rate against demand from last spring, last summer, and last fall — all periods with completely different seasonal patterns. For a seasonal product, that average is nearly meaningless.
Use 4–8 recent weeks as your baseline sell rate. Adjust for seasonality if you have it — a style that normally peaks in week 40 should be evaluated against what its expected sell rate this time of year is, not its all-time average.
The seasonality trap: A style might show 18 weeks of cover in October using a simple trailing average. But if it's outerwear and you're heading into peak season, the forward-looking demand is much higher — real coverage might be 6–8 weeks. Always ask: "Is my sell rate about to change?"
On-hand inventory is what's in your warehouse today. But you likely have units on order that will arrive in the next few weeks. A complete WOC calculation uses available inventory, not just current on-hand:
If you have 200 units on hand and 150 on order arriving in 3 weeks, your real coverage picture is different than what on-hand alone tells you.
A style might show 12 weeks of cover overall, but size M might be down to 2 weeks while size L has 24 weeks. The aggregate looks healthy while you're about to lose sales in a key size.
This is why size-level WOC tracking matters for apparel. Style-level cover is a useful first pass, but SKU-level cover catches the problems that style-level masks.
There's no universal target. The right WOC depends on your lead time, your margin structure, and whether the style is a core replenishment item or a fashion newness item with a defined end date.
| WOC vs. Lead Time | Signal | Action |
|---|---|---|
| WOC < Lead time | Critical — stockout imminent | Place emergency reorder immediately; consider air freight |
| WOC = Lead time to Lead time + Safety stock | Watch — reorder now | Place replenishment order this week |
| WOC = Lead time + Safety stock to 2x Lead time | Healthy | No action needed; monitor weekly |
| WOC > 2x Lead time (for end-of-season items) | Possible over-buy | Evaluate markdown strategy to clear before end date |
A useful heuristic for fashion retail: you want WOC to be lead time + safety stock buffer for replenishment styles, and trending down toward zero by end-of-season date for non-replenishment styles.
Weeks of cover by itself is only half the picture. The other half is lead time — how long it takes to get more inventory.
If your vendor lead time is 8 weeks and a style has 9 weeks of cover, you have exactly one week to decide before you run out of runway to place a reorder. That's an emergency even though the inventory looks nominally adequate.
The common mistake: Buyers look at WOC, see a number higher than zero, and feel safe. The right question is always: "Is my current WOC higher than my lead time?" If not, you're already late.
This is why demand planning tools express reorder alerts as a function of lead time, not just raw WOC. A style at 4 weeks of cover with a 2-week lead time is fine. The same style with a 6-week lead time is a stockout in progress.
For seasonal items, WOC needs to be evaluated against your end date, not just your lead time. A style going out of season in 8 weeks with 18 weeks of cover at the current sell rate is going to land at clearance unless something changes.
End-of-season WOC analysis typically works backwards from the planned off-shelf date:
If current sell rate is below required rate, you have a sell-through problem — the WOC is too high relative to the time remaining. That's when markdowns, promotional placement, or allocation shifts become the right conversation.
The buyers who get the most value from WOC use it as a prospective discipline, not a retrospective report. Instead of reviewing WOC weekly to see what's broken, they set WOC targets for each style category at the start of a season — "I want outerwear at 14 weeks of cover entering October, 8 weeks entering November, and 4 weeks exiting December" — and then actively manage inventory levels toward those targets.
This is the difference between reactive buying ("we just stocked out, emergency order") and proactive buying ("we'll stock out in 3 weeks if we don't act today"). Same metric, different cadence and mindset.
WOC is most powerful when it's integrated with your other planning metrics rather than tracked in isolation:
A demand planning tool that surfaces all of these together — rather than requiring you to triangulate between separate spreadsheets — is where WOC becomes a real operational advantage rather than a number you look up once a week.
Reactive SDP calculates seasonally-adjusted WOC for every style, automatically surfaces reorder urgency, and generates a purchase order you can send to your vendor the same day.
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