Weeks of Supply: The Formula, a Worked Example, and How to Set Targets
Weeks of supply is one number that tells you how long your current inventory will last if demand keeps up. Get it right and you stop guessing about reorders. Get it wrong and you either run out of your best sellers or park cash in stock you can't move fast enough.
It's a forward-looking metric, not a rear-view one. Sales reports tell you what already happened. Weeks of supply tells you how many weeks of runway you have left before a SKU hits zero. That's the number a buyer actually acts on.
What weeks of supply actually measures
Weeks of supply (usually shortened to WOS) answers a single question: at your current sales rate, how many weeks until you run out of this product?
If a SKU has 8 weeks of supply, you have roughly two months of runway at today's pace. If it has 1 week, you're about to stock out and you probably already missed the window to reorder. If it has 40 weeks, you overbought, and that cash is stuck on a shelf.
The metric is most useful per SKU, per location. A single blended number across your whole catalog hides the products that are about to run dry and the ones quietly eating your working capital.
The weeks of supply formula
The core formula is short:
Weeks of supply = Inventory on hand / Average weekly demand
Two inputs, that's it:
- Inventory on hand. The units you physically have available to sell right now.
- Average weekly demand. How many units you sell per week, on average.
Some teams include inbound stock (units already on a purchase order) in the numerator to get a fuller picture. That's fine, just be consistent and label it clearly, because "on hand" and "on hand plus on order" are two different numbers that answer two different questions.
A worked example
Say you sell a wireless keyboard.
- You have 1,200 units on hand.
- You sell about 150 units per week.
Plug it in:
Weeks of supply = 1,200 / 150
Weeks of supply = 8 weeks
So you're carrying 8 weeks of runway. Whether that's healthy depends entirely on your lead time, which is the next piece.
Weeks of supply vs days of supply vs inventory turnover
These three metrics measure the same underlying thing (how fast stock moves relative to how much you hold), just on different scales. They convert cleanly:
| Metric | Formula | Our example | Best for |
|---|---|---|---|
| Weeks of supply | On hand / weekly demand | 8 weeks | Reorder decisions, medium lead times |
| Days of supply | Weeks of supply x 7 | 56 days | Fast movers, short lead times |
| Inventory turnover | 52 / weeks of supply | 6.5 turns/year | Finance, comparing across catalog |
A quick sanity check on that last row: 8 weeks of supply means you cycle through your stock about 6.5 times a year (52 weeks divided by 8). Lower weeks of supply means higher turnover, which is generally more efficient use of cash, right up until it tips into stockouts. That tension is the whole game.
How to set a target weeks of supply
The right target isn't a round number someone picked in a meeting. It's driven by how long restocking takes and how much risk you want to carry. Build it up in steps.
- Start with your lead time in weeks. How long from placing a purchase order to having the stock sellable. Say the keyboard supplier takes 5 weeks.
- Add your review cycle. If you only check stock and place orders once a week, add 1 week, because a SKU can drift for up to a week before you notice.
- Add safety stock in weeks. A buffer for weeks demand runs hot or the supplier runs late. Say you want 2 weeks of cushion.
- Add them up for your reorder trigger. 5 + 1 + 2 = 8 weeks. When this SKU drops to 8 weeks of supply, you place an order.
- Set an upper bound too. Add your order cycle (how often you reorder) on top of the trigger to get your "order up to" ceiling. If you reorder every 4 weeks, your target sits around 12 weeks right after a delivery lands and drifts down to 8 before the next order.
That gives you a working range instead of a single fragile number: reorder near 8 weeks, top out near 12, and treat anything sliding under your safety buffer as a fire.
Reading the signal
Once you're tracking weeks of supply per SKU, the number does most of the talking.
Too low (below your reorder trigger). You're inside the danger zone where a normal demand spike causes a stockout before the next shipment arrives. Reorder now, and if it's chronic, either raise safety stock or find a faster supplier.
Too high (well above your ceiling). Cash is trapped. Common culprits are a demand forecast that came in soft, a bulk buy that overshot, or a product past its peak. Options: slow or pause reorders, run a promotion, or move stock to a location that's actually selling it.
Steady inside the range. This SKU is behaving. Leave it alone and spend your attention on the outliers.
The trap to avoid is watching one blended average for the whole business. Healthy on average can still hide a dozen SKUs about to run dry and a dozen more you badly overbought. They cancel out in the average and cost you on both ends.
Track it where your stock actually lives
Weeks of supply only works if the two inputs are current. On-hand counts drift the moment they're stale, and weekly demand shifts with seasonality. A spreadsheet you update by hand is fine for ten SKUs in one place. It falls apart across hundreds of SKUs in three warehouses that each sell and restock at different speeds.
This is where an inventory system earns its keep. Inventoros tracks on-hand stock across every location in real time, so the numerator in your weeks-of-supply math is always live, and it's open source and self-hosted, so you own the data. You can read current stock and movement straight from the REST and GraphQL API to compute weeks of supply in your own dashboards or feed it to a reorder script. The install guides cover cPanel, a VPS, and Docker if you want to run it yourself.
FAQ
What is a good weeks of supply number? There's no universal target. A good weeks of supply covers your full lead time plus a safety buffer, and no more. Short lead times and fast movers might sit healthy at 2 to 4 weeks. Long overseas lead times might need 10 or more. The right number is the one that clears your reorder-plus-safety math without piling on excess.
How is weeks of supply different from days of supply? Same metric, different time scale. Days of supply is just weeks of supply times seven. Retailers with fast lead times often prefer days for finer resolution. Anyone ordering on weekly or monthly cycles usually finds weeks easier to reason about. Pick one unit and stick with it so nobody confuses a "5" that means days with a "5" that means weeks.
Should I use past sales or a forecast for weekly demand? For stable products, a trailing average (say the last 8 to 12 weeks) is simple and reliable. For seasonal or trending items, a trailing average lags reality and will steer you wrong, so use a forward forecast instead. If sales are climbing into a season, backward-looking demand understates your real burn rate and quietly sets you up for a stockout.
Can weeks of supply predict stockouts? Yes, that's its main job. When a SKU's weeks of supply drops below your lead time plus review cycle, you're on track to run out before a fresh order can land. Watching that threshold per SKU gives you an early warning while there's still time to act, which is exactly what a raw stock count can't do on its own.