What is weeks of supply?
Weeks of supply (WoS) is an inventory planning metric that answers one practical question: at the current rate of sales or usage, how many weeks will this inventory last?
It is used across retail, e-commerce, wholesale, manufacturing, and distribution because it converts a unit count into an actionable time horizon. Instead of knowing you have 2,400 units in stock, you know you have 8 weeks of runway — which directly informs when to reorder, how much to order, and whether you are overstocked or understocked relative to lead time.
Two important things to understand before calculating:
- WoS is a snapshot, not a forecast. It assumes current demand continues at the same rate. Seasonal shifts, promotions, or trend changes will alter the true coverage significantly.
- Higher WoS is not always better. Excess inventory ties up capital, increases storage costs, and raises obsolescence risk. The goal is the right amount — not the most.
Weeks of supply formula
Two inputs — both must use the same unit:
- Current inventory — the number of units on hand today, available for sale or production. Use actual on-hand stock, not units on order.
- Average weekly demand — typical units sold or consumed per week. You can use historical weekly sales average, a rolling 4–12 week average, or a forward demand forecast. Using a forecast is more appropriate when demand is changing.
Converting from monthly or annual demand
If you only have monthly or annual demand figures, convert first:
Interpreting weeks of supply — inventory zones
The right WoS target depends on your lead time, supplier reliability, and demand variability. A simple framework divides inventory health into four zones:
For example, if your supplier lead time is 3 weeks and you target 2 weeks of safety stock, your reorder point is at 5 weeks of supply. Anything below 5 WoS means you should already be placing an order. Anything above 12–14 WoS likely represents excess that is tying up capital.
Step-by-step calculation method
Worked examples
Grocery store SKU
On hand: 900 units · Avg weekly demand: 100 units
✅ 9 weeks — healthy if lead time is 4–6 weeks
Same stock, faster demand
On hand: 2,000 units · Demand jumps to 500/week
🟡 4 weeks — reorder now if lead time is 3+ weeks
Component buffer
Components: 3,600 · Weekly usage: 450 in production
✅ 8 weeks — solid buffer for most supply chains
Planning ahead for peak season
On hand: 1,250 · Forecast: 350/week during peak (up from 250)
⚠️ At peak demand, coverage drops to 3.6 weeks — order early
How demand level changes WoS — same 2,400 units
This shows the direct inverse relationship between demand and weeks of supply:
Weeks of supply vs inventory turnover — two sides of the same coin
Weeks of supply and inventory turnover measure the same underlying relationship — how quickly inventory moves relative to demand — from opposite angles:
A business with 6.5× annual inventory turnover effectively carries about 8 weeks of supply at any given time. Using both metrics together gives a more complete picture — WoS for day-to-day operations and turnover for performance benchmarking.
Common mistakes to avoid
- Using outdated or unrepresentative demand data. A 12-month average demand will be misleading if your product has strong seasonality. Use a rolling window that reflects the period you are planning for.
- Including inbound inventory (open POs) in current stock. WoS should use what is physically on hand. Units on order that haven't arrived should be tracked separately as pipeline inventory.
- Mismatching units. Inventory in cases divided by demand in individual units — or vice versa — will give a nonsense result. Confirm both inputs use the same unit of measure.
- Not comparing WoS against lead time. A WoS of 4 weeks looks fine until you learn your lead time is 5 weeks. Always interpret WoS relative to your replenishment timeline and safety stock requirement.
- Treating high WoS as always good. Excess inventory increases carrying costs, storage charges, and the risk of obsolescence or expiry. The optimal WoS is the minimum that covers lead time plus safety stock — not the maximum.
- Ignoring demand variability. If weekly demand swings significantly, average demand understates the true risk. High demand variability requires more safety stock — and therefore a higher target WoS — than stable demand at the same average.
Frequently asked questions
What is the formula for weeks of supply?
Weeks of Supply = Current Inventory ÷ Average Weekly Demand. Both inputs must use the same unit of measure. If you have monthly demand, divide by 4.33 to get weekly demand first.
What is a good weeks of supply target?
It depends entirely on your lead time, safety stock policy, and demand variability. A common rule of thumb is lead time plus 1–2 weeks of safety stock. For a supplier with a 3-week lead time and moderate demand variability, a target of 5–8 weeks of supply is typical. Fast-fashion retailers may target as low as 2–4 weeks; industrial manufacturers may carry 12+ weeks.
How is weeks of supply different from inventory turnover?
They measure the same relationship from opposite angles. Weeks of supply tells you how many weeks stock will last (lower = faster moving). Inventory turnover tells you how many times per year inventory is replaced (higher = faster moving). The conversion is: WoS = 52 ÷ Annual inventory turnover.
Can weeks of supply be too high?
Yes. Excess inventory ties up working capital, increases storage and insurance costs, and raises the risk of obsolescence, expiry, or mark-down pressure. High WoS relative to your target range is a signal to slow purchasing, promote the item, or investigate whether demand has shifted downward.
Should I use historical demand or forecasted demand?
Use forecasted demand when it is available and reliable — especially ahead of seasonal peaks, promotional events, or known demand changes. Historical demand works well for stable items with no upcoming changes. For items with high seasonality, using a simple historical average will systematically overstock in slow periods and understock in peak periods.