Days of cover vs weeks of supply vs runway: three time metrics, one division
Days of cover is stock ÷ daily velocity — but weeks of supply and runway answer different questions. Formulas, when each metric lies, and a comparison table.
InventoryX is in development and not available today. ClearStock is the live Shopify app.
Days of cover is how many days your current stock lasts at the current selling rate: days of cover = sellable units on hand ÷ average daily units sold. It's the most actionable inventory metric there is, because it converts a count into a deadline. But it's only one of three time-based inventory metrics merchants use interchangeably, and they are not interchangeable: days of cover ignores what's already on the water, weeks of supply is the same number in a coarser unit, and runway is the number that includes inbound stock. Confusing them is how a SKU at "12 days of cover" gets treated as an emergency when 400 units are arriving Thursday.
Here's what each one actually measures, the table that keeps them straight, and the specific situations where each one misleads.
The base calculation, and the denominator that decides everything
A ceramic mug, MUG-GRN-12:
- Sellable on hand: 171 units
- Units sold, last 90 days: 559
- Days in stock during those 90: 79
Corrected velocity: 559 ÷ 79 = 7.08 units/day Days of cover: 171 ÷ 7.08 = 24.2 days
Now the uncorrected version, the one a report will hand you: 559 ÷ 90 = 6.21 units/day, giving 171 ÷ 6.21 = 27.5 days. Three extra days of apparent comfort that don't exist. That's the whole failure mode in one line — an understated velocity overstates cover, and overstated cover is the direction that delays a purchase order. The mechanics of the correction, including why the zeros cluster around your best weeks, are in the in-stock-only velocity correction.
Two more denominator traps worth knowing before you compute anything:
- Sellable means sellable. On-hand in Shopify includes units that are committed to open orders and units sitting at a location that doesn't fulfill online orders. Available is the number that can actually sell; that's the numerator.
- Use the right window. 90 days smooths one promo. 30 days reacts to a trend. For a seasonal SKU, the same 90 days last year beats the last 90 days, scaled to your growth.
The three metrics side by side
| Metric | Formula | Unit of decision | Answers | Ignores |
|---|---|---|---|---|
| Days of cover | sellable on hand ÷ daily velocity | Days | "How long until this SKU is empty if nothing changes?" | Inbound stock, cash, demand shifts |
| Weeks of supply | days of cover ÷ 7 (or on hand ÷ weekly velocity) | Weeks | The same question, at a reporting grain | Everything days of cover ignores, plus a week of resolution |
| Runway | (sellable on hand + confirmed inbound) ÷ daily velocity | Days | "How long until I'm empty even counting what's coming?" | Inbound that slips, inbound that's already sold |
The distinction that matters operationally: days of cover is a trigger metric, runway is a comfort metric. Cover tells you whether a decision is due. Runway tells you whether the decision is already too late. A SKU with 12 days of cover and nothing inbound needs a PO today; the same 12 days with 300 units landing in six days needs a calendar note.
When each metric lies
Days of cover lies when you have inbound stock. It's a snapshot of what's on the shelf, not a forecast of what will be available. A store that reacts to cover alone will double-order — placing a PO against a low cover number while a container is already three weeks out. Always read cover and inbound together, and treat runway as the number you quote in a panic.
Days of cover also lies when the numerator includes unsellable units. Damaged stock, stock at a location excluded from online fulfillment, units reserved against wholesale orders, and preorder allocations all inflate on-hand without being available. The metric is only as honest as the filter behind it.
Weeks of supply lies at decision boundaries. At 7-day resolution, 20 days and 27 days of cover both read as "3 weeks." On a 45-day lead time both are urgent, so the rounding rarely changes the decision — but on a 10-day domestic supplier, 8 days versus 13 days is the difference between ordering today and ordering next week, and both round to "1–2 weeks." Use weeks for reporting, days for math. Mixing the two is the classic error: a "6-week lead time" compared against "8 weeks of supply" that was actually computed as 56 days of stock against 42 days of transit.
Runway lies when inbound dates are optimistic. Runway is only as good as the arrival dates in it. Suppliers quote dates; ports and customs renegotiate them. If your POs carry expected dates rather than confirmed ship dates, runway is a hope with a decimal point. It also lies in the opposite direction when inbound is already sold — units committed to wholesale or preorders aren't available to cover retail demand.
All three lie when velocity is stale. Cover, weeks of supply, and runway all divide by the same number. If that number came from calendar days, or from a quarter that included a promo, all three metrics move together in the wrong direction and no amount of cross-checking between them will catch it.
Reading days of cover against your lead time
Cover has no absolute good value. It only means something next to the clock that replenishes it:
| Days of cover | vs. 45-day lead time | Reading |
|---|---|---|
| 0–20 | Far below | Already late. Order today; treat the gap as a planning failure to review, not a surprise. |
| 21–45 | Below | The PO window is open. Every day of delay converts to roughly a day of future stockout. |
| 46–52 | At lead time | Textbook trigger point — order now, arrive near zero, no buffer. |
| 53–75 | Lead time + buffer | Healthy. This is where a correct reorder point should have fired. |
| 76–120 | Well above | Comfortable, or a slow mover forming. Check sell-through before feeling good. |
| 120+ | Months of cover | Dead-stock risk, not safety. |
Rescale the bands to your own clock: 20 days of cover is comfortable on a 7-day supplier and catastrophic on a 45-day one. This is why comparing cover across SKUs — "mugs are at 24 days, candles at 80, candles are fine" — is meaningless without each SKU's lead time beside it.
Computing it across a catalog
Shopify's "Inventory remaining per product" report does compute days of inventory remaining, and it corrects velocity for active days — but it uses a fixed 28-day window, returns N/A whenever a variant had zero sales, and cannot be filtered to your lead time. So most stores still recompute it, in one of two places:
- A spreadsheet. Export on-hand quantities, export 90-day units per SKU, add your dark-day notes, one column:
=on_hand/(units/days_available). An hour for a full catalog if your history is clean. - An app that already computes per-SKU velocity. ClearStock, our live Shopify app, calculates sales velocity and days of cover from your Shopify data and uses them to classify products as moving, slowing, stuck, dead, or oversold — it's a slow-and-dead-stock tool rather than a buying planner, and it doesn't forecast or draft purchase orders. ClearStock uses a fixed review window and does not currently exclude out-of-stock days from velocity; if that correction matters to you, it has to be made by hand or by a tool that explicitly does it. That single assumption swings the number by 20–30% on any SKU that has been dark recently.
Then put the number to work on a schedule rather than a whim: sort the catalog by cover ascending, filter to SKUs whose cover is below their lead time, and that's your buying list. Nothing fancier is required, and doing it weekly beats doing it perfectly once.
Common questions
What's the difference between days of cover and weeks of supply? They're the same division in different units: weeks of supply is days of cover ÷ 7. Weeks is the common reporting grain in retail planning; days is the operating grain, and the one to use for reorder math, because lead times arrive in days and rounding to weeks blurs decisions on short lead times.
What's the difference between days of cover and runway? Days of cover counts only sellable on-hand stock. Runway adds confirmed inbound, so it answers "when am I actually empty?" rather than "when is my shelf empty?" Cover is your trigger metric; runway is your comfort metric. Quoting one when someone means the other is how a well-stocked SKU gets panic-ordered.
What is a good days of cover? There isn't a universal figure — cover should sit at roughly lead time plus buffer days for that SKU. On a 45-day supplier that's 52–75 days; on a 7-day supplier it's 10–14. Comparing cover between SKUs with different lead times tells you nothing.
Should days of cover include incoming inventory? No — that's runway. Keep them separate, because they answer different questions and get used in different conversations. Days of cover drives the reorder decision; runway tells you whether the decision is already late.
Why is my Shopify days of cover number different from an app's? Almost always the velocity denominator. If one calculation divides units sold by calendar days and the other by days the SKU was actually available, they'll disagree by roughly the stockout share of the window — 15–25% on a SKU that ran dry. Check which one excludes dark days, and also check whether on-hand or available stock is the numerator.
Next step
Compute days of cover for your ten fastest SKUs using available stock and in-stock-only velocity, then add a second column for runway that includes confirmed inbound. The SKUs that are low on cover and have nothing inbound are the only ones that need a decision today — and for those, the trigger math that would have caught them earlier is in the reorder point formula.