Weeks of supply
Max Pechonis · Founder, Rebridge ·
Weeks of supply is how long your current stock will last at the rate it is currently selling. Units on hand divided by average weekly unit sales. It converts a stock level into a length of time, which is the form a buying decision actually needs.
The formula
Weeks of supply = units on hand ÷ average weekly unit sales
A style with 180 units on hand selling 15 a week has 180 ÷ 15 = 12 weeks of supply.
The value of the conversion is that it puts stock into the same unit as everything else you are deciding about. A vendor lead time is in weeks. A season is in weeks. "180 units" answers nothing on its own; "12 weeks, and the season has 9 left" answers the question immediately.
The averaging window is the whole difficulty
Weeks of supply is only as good as the sales rate you divide by, and choosing that rate is a judgement rather than a calculation.
Too short a window and one good weekend makes a slow product look urgent. Too long and a range that has genuinely stopped selling looks healthy on the strength of how it started. Four to eight weeks is the usual compromise for products with steady demand.
The window fails completely on anything seasonal, because a trailing average is a measure of the past and a seasonal product's next four weeks do not resemble its last four. Coats in August and coats in November share a sales rate only in the arithmetic. For those, the comparison that works is the same weeks last year rather than the weeks just gone.
Reading it against lead time
The number becomes a decision when you set it beside how long a reorder takes to arrive.
Weeks of supply below the lead time means you will run out before a reorder can land, whatever you do today. Roughly equal means you are ordering at exactly the right moment and have no margin for the delivery slipping. Comfortably above means you have room, and the question becomes whether the season has enough weeks left to sell what you already hold.
That last case is the one people miss. Twelve weeks of supply is healthy in March and is dead stock in a range that finishes in six.
Questions
What is the difference between weeks of supply and weeks of cover?
Nothing. They are the same calculation under two names, used interchangeably, and both get shortened to WOS or WOC in reports. If a report gives you one and a colleague asks for the other, they are asking for the same number.
Should weeks of supply include stock on order?
State which version you are using, because both are useful and they answer different questions. On-hand only answers whether you will run out. On-hand plus on-order answers whether you have over-committed. Reports that silently mix them are how a buyer orders into an overstock.
How does weeks of supply relate to stock-to-sales ratio?
They measure the same thing on different clocks. Stock-to-sales is monthly, so a ratio of 3.25 is roughly 14 weeks of supply, since a month averages about 4.33 weeks. Weeks of supply is the more natural unit when you are comparing against a lead time.