The sell-out countdown for a SKU: the number to check against your supplier’s lead time before every reorder.
Weeks of supply = units on hand ÷ average weekly unit sales. It converts a stock count into a deadline. With 1,200 units on the shelf selling 150 a week, you hold 1,200 ÷ 150 = 8 weeks of supply, about 56 days until sell-out at the current pace. The reading only matters next to your lead time: 8 weeks of supply against a 6-week resupply cycle means reordering within a fortnight, while the same 8 weeks against a 1-week domestic supplier is comfortable overstock. Enter units and weekly sales per SKU to get the countdown.
Weeks of supply
8
Days of supply
56
Estimate only. Results reflect exactly the numbers you enter — verify
against your own accounting before making pricing decisions.
8 weeks of cover: time to reorder calmly against a 6-week lead time.
Bestseller running hot
Units on hand
450
Average weekly sales
180
Weeks of supply
2.50
Days of supply
17.50
2.5 weeks left: past the reorder point for anything but same-week resupply.
The weeks of supply formula. The weeks of supply formula is units on hand ÷ average weekly sales. That is the whole calculation: if you want to know how to calculate weeks of supply for any SKU, take the sellable units in stock today and divide by the units a typical week sells. 1,200 on hand at 150 a week is 8 weeks. The same metric appears on retail reports as weeks of inventory, weeks of cover, or WOS; all of them name this one division. Its dollar-based sibling is days of inventory, computed from COGS for the store as a whole.
Is 6 months of inventory too much? Six months is roughly 26 weeks of supply, and for most SKUs that is far more cover than any lead time justifies. Half a year of inventory storage is never free: warehouse or 3PL rent runs the whole time, and Amazon FBA adds aged-inventory surcharges once units cross 181 days. Price the bill with the Amazon storage fee calculator or your own rate in the inventory carrying cost calculator; at a typical 25% annual carrying rate, a 6 month hold quietly consumes about 12% of the stock’s value. A deep buy can still win — a once-a-year sea shipment, a pre-tariff order — but only when the supplier discount beats that storage math.
Frequently asked questions
How many weeks of supply should I keep?
Your supplier lead time in weeks, plus a safety buffer of 1–3 weeks depending on how volatile the SKU’s demand is. Importing on an 8-week cycle means holding 9–11 weeks; a domestic 1-week supplier needs only 2–3. Holding much more than lead time plus buffer is cash parked without purpose; holding less is a scheduled stockout.
Which sales average should I use for weekly sales?
A trailing 4-week average is the standard for stable SKUs: long enough to smooth daily noise, short enough to track trend. For fast-trending or promotional products, weight recent weeks more heavily or use the last 2 weeks; for seasonal items, use last year’s same-season rate adjusted for growth. Whatever you pick, exclude stockout weeks: zero sales weeks from being out of stock poison the average and inflate the answer.
How is weeks of supply different from days of inventory?
Weeks of supply is the per-SKU operational version, computed in units for reorder timing. Days of inventory (DIO) is usually the store-wide financial version, computed in dollars from COGS for cash-flow and health analysis. A buyer decides "reorder SKU-142 this week" from weeks of supply; a founder decides "we are over-invested in inventory" from DIO. Same concept, different altitude.
What should trigger action from this number?
Set two thresholds per SKU. Reorder when weeks of supply falls to lead time plus your safety buffer: that is the moment to place the PO, not to start thinking about it. Flag overstock when supply exceeds roughly twice that level; those SKUs should skip their next reorder cycle and may need promotion. Reviewing both lists weekly takes minutes and prevents most stockouts and most dead stock.
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