The formula
Weeks of Supply = Units on Hand ÷ Average Weekly Unit Sales
If you hold 240 units of a style and it sells an average of 30 units a week, you have 8 weeks of supply. Multiply by seven for days of supply (56 days), which is easier to compare against a lead time quoted in days.
Trailing vs. forward weeks of supply
Trailing weeks of supply uses recent average sales — simple and good for stable sellers. Forward weeks of supply divides on-hand by forecast weekly demand, which matters when you’re heading into or out of a season and last month’s rate would mislead you. A swimwear style with 10 weeks of supply in March may have one real week of supply once summer demand hits.
Why it’s the buyer’s reorder trigger
The decision is a direct comparison:
If Weeks of Supply ≤ Lead Time + Safety Buffer → reorder now
If a vendor takes four weeks to deliver and you want two weeks of safety stock, then anything at or below six weeks of supply needs a PO today — otherwise you’ll stock out before the next shipment lands. That’s the link between weeks of supply and your reorder point.
Healthy ranges
There’s no universal target — it’s driven by lead time, MOQs, and category rhythm:
- Core replenishment items: often kept tight, just above lead time, to protect turns.
- Seasonal goods: high weeks of supply early in the season, deliberately driven toward near-zero by the end.
- Long-lead or imported goods: naturally carry more weeks of supply because the reorder pipe is slow.
Too many weeks of supply ties up cash and invites markdowns; too few risks stock-outs on your best sellers. The art is matching it to lead time.
Weeks of supply and turns
Weeks of supply is the forward-looking cousin of inventory turnover. As a rough identity, average weeks of supply ≈ 52 ÷ annual turns. Eight weeks of supply across the year is about 6.5 turns. If you manage weeks of supply well at the SKU level, healthy turns follow at the aggregate.
Weeks of supply in Vendee Pro
Vendee Pro includes a Weeks-on-Hand report in Report Studio, and reorder automation uses sell-through and demand signals to propose vendor-grouped draft POs you confirm with one click — so the “am I running out before the next delivery?” question is answered before it becomes a stock-out. Inventory feature →
Frequently asked questions
What is weeks of supply?
Weeks of supply (also called weeks on hand) is the number of weeks your current inventory will last at the current average sales rate. It’s calculated as inventory on hand divided by average weekly unit sales.
How do you calculate weeks of supply?
Divide units on hand by average weekly unit sales. If you hold 240 units and sell an average of 30 per week, you have 8 weeks of supply. Multiply by 7 for days of supply.
What is a good weeks-of-supply target?
It depends on lead time and category. A useful rule is to keep weeks of supply above your replenishment lead time plus a safety buffer. Core, fast movers often run leaner; seasonal items carry more early and less near the end.
How is weeks of supply different from inventory turnover?
They’re two views of the same thing. Turnover counts how many times stock sells through in a year; weeks of supply is a forward snapshot of how long current stock lasts. Roughly, weeks of supply ≈ 52 ÷ annual turns.
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