Weeks of supply: formula, example and how to set a target

Weeks of supply tells you how long your stock will last at the rate it is selling. It is one of the simplest inventory measures, and one of the most useful: it turns a quantity on the shelf into time, which everyone in the business understands.

The weeks of supply formula

Weeks of supply = stock on hand ÷ average weekly sales

Use units for both, or value at cost for both. For a single item, units are easier to read. For a category or a whole store, use value, so a case of screws and a pallet of motors are not added together.

A worked example

An item has 600 units on hand. It is forecast to sell 75 units a week over the next few weeks.

Weeks of supply = 600 ÷ 75 = 8 weeks.

Now suppose you used the last 13 weeks of sales instead, when it sold 780 units, or 60 a week. That gives 600 ÷ 60 = 10 weeks. If the item is speeding up, the history overstates how long the stock will last by two weeks, which is often the difference between ordering now and running out.

On hand or inventory position?

Weeks of supply on hand counts only what is on the shelf. Add what is already on order and you get the cover of your inventory position: with 300 more units on order, 900 ÷ 75 = 12 weeks. Use the first to spot what is about to run out, and the second to decide whether to order more.

Weeks of supply and inventory turnover

Weeks of supply ≈ 52 ÷ inventory turnover

Stock that turns 6.5 times a year sits about 52 ÷ 6.5 = 8 weeks on average. The two say the same thing in different ways, as long as both are measured at cost over the same period. See what counts as a good inventory turnover ratio, or work it out with the inventory turnover calculator.

How to set a target

The right weeks of supply depends on how often you order and how much buffer you need.

Point in the cycle	Weeks of supply
Just before an order arrives	About your safety stock in weeks
Just after an order arrives	Safety stock plus the weeks until the next order
On average	Safety stock plus half the weeks between orders

For an item ordered every 4 weeks with 1.5 weeks of safety stock, cover should swing between about 1.5 and 5.5 weeks, and average about 3.5. Well above that range, the item is overstocked. Below it, it is at risk. Size the buffer with the safety stock formula.

In retail: by category and by season

• Compare categories, not only the total. A store with 10 weeks of supply in total can be out of its best sellers and buried in last season’s stock.
• For seasonal items, compare the weeks of supply with the weeks left in the season. Stock that will outlast the season needs a markdown or a transfer now.
• Plan purchases with it. Open-to-buy turns a target weeks of supply into a budget for what you can still buy. Try the open-to-buy calculator.
• Read it with sell-through. High weeks of supply and a low sell-through rate together mean the item is not selling.

Common mistakes

• Dividing by a year’s average for a seasonal item. Use the coming weeks.
• Counting stock that cannot be sold: damaged, on hold or promised to a customer.
• Ignoring items with no sales. Their weeks of supply is infinite: list them separately as dead stock.
• Mixing units and value in the same report.

Weeks of supply for every item

Store health in the Statadventure Optimization Lab shows weeks of supply, sell-through and GMROI for every item in every store, from your point-of-sale exports. See retail inventory management software.

Frequently asked questions

What is a good weeks of supply?

Enough to cover the time until the next order arrives plus your safety stock, and not much more. For an item ordered every four weeks with one and a half weeks of safety stock, roughly 1.5 to 5.5 weeks is healthy.

What is the difference between weeks of supply and days of supply?

Only the unit. Days of supply is weeks of supply × 7, or the stock on hand ÷ average daily sales.

Should weeks of supply use past sales or a forecast?

A forecast of the coming weeks, when you have one. Past sales are fine for steady items, but they overstate cover for an item that is slowing down and understate it before a busy season.