Weeks of Supply Calculator
Calculate how many weeks your current inventory will last at the current demand rate. Add incoming inventory, safety stock, and lead time to see net usable coverage, reorder zone status, coverage vs lead time visual, and a demand scenario comparison showing WoS at different demand rates.
Enter your inventory data
Start with on-hand units and weekly demand — the optional fields add operational context to make the result more actionable.
Core formula
WoS = Inventory ÷ Weekly demand
With adjustments:
Net units = On hand + Incoming − Safety stock
WoS = Net units ÷ Weekly demand
WoS vs lead time
Always compare WoS against your supplier lead time plus safety buffer. If WoS ≤ lead time you are already behind — an order placed today won't arrive until after you've stocked out.
Frequently asked questions
What is weeks of supply?
Weeks of supply (WoS) is the number of weeks your current inventory will last at the current demand rate. It converts a unit count into a time horizon — making reorder timing, stockout risk, and excess inventory much easier to act on than raw unit counts alone.
Should I include incoming inventory in the calculation?
Include incoming inventory if it is confirmed and scheduled to arrive before a stockout would occur. Do not include orders that are speculative or at risk of delay. Overestimating incoming inventory will give a false sense of coverage.
Is a higher weeks of supply always better?
No. Excess inventory ties up working capital, increases storage and insurance costs, and raises the risk of obsolescence. The optimal WoS is the minimum that covers lead time plus a safety buffer — not the maximum.
How does weeks of supply relate to inventory turnover?
They measure the same relationship from opposite directions. WoS = 52 ÷ Annual inventory turnover rate. A business turning inventory 6.5 times per year carries roughly 8 weeks of supply on average.
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Disclaimer
This calculator is for educational and planning purposes only. It does not provide accounting, logistics, purchasing, or financial advice. Real inventory planning requires item-level demand forecasts, service-level targets, supplier reliability data, and more detailed operational analysis.