Weeks of Supply Calculator (WOS)
How many weeks the stock on hand will last at the current rate of sale.
The unit buyers actually plan in. It answers 'will this last until the next delivery?' in one number.
- Formula
- Units on Hand ÷ Average Weekly Unit Sales
- Benchmark
- Specialty and fashion 8–12 weeks, basics 12–16 weeks; slower-turning department-store and hardline assortments plan materially more cover
Worked example
- Units on hand
- 480
- Average weekly sales (units)
- 60
- Result
- 8.0 weeks
- Note
- Healthy
How to read it
Weeks of supply is how long the stock on hand lasts at the current rate of sale — a result of 8.0 means eight weeks if nothing changes. The judgement is always against lead time. Eight weeks of cover with a six-week lead time is comfortable; the same eight weeks against a twelve-week lead time means you will be out of stock before the replacement lands. The label the calculator shows is a rule of thumb that ignores lead time — your lead time overrides it.
Common mistake
Using a single week's sales as the rate of sale. One promotion, one quiet week, one Eid weekend, and the figure swings wildly. Use a rolling four-week average — and when demand is genuinely trending, use the trend rather than the average, because a line growing 10% a week will run out sooner than a flat average suggests.
What to do about it
Above sixteen weeks you are funding stock that is not working: stop replenishment before you discount, because a markdown on a line that is still being reordered solves nothing. Below six weeks, check the lead time before you treat a lean position as a good one — if the replacement cannot land in time, the lean number is a stockout waiting to happen.
Questions
How many weeks of supply should I hold?
Enough to cover the lead time plus a safety buffer. Specialty and fashion lines commonly sit at 8–12 weeks and basics at 12–16, while slower-turning department-store and hardline assortments plan materially more cover — but it is the lead time that decides it, not the category label.
Should weeks of supply use units or value?
Units for replenishment decisions, value for cash planning. Mixing them is a common source of arguments between buying and finance, because a position that looks lean in units can still be heavy in value if the expensive lines are the slow ones.
What if a line has no sales history?
Weeks of supply cannot tell you anything useful on a new line. Use the planned rate of sale from the buy, and replace it with the real one after three or four weeks of trading.