Weeks of Supply: Formula, Examples and How to Use It

Weeks of supply estimates how many weeks current inventory can support expected demand. It converts stock quantity into a coverage measure that is easier to compare across products with different sales volumes.

A simple formula is:

Weeks of Supply = Available Inventory ÷ Average Weekly Demand

If 240 units are available and average demand is 40 units per week, the business has approximately six weeks of supply.

Why weeks of supply is useful

Knowing that an item has 500 units on hand does not tell you whether that is a lot or a little. Five hundred units may represent two days of demand for one product and six months for another.

Weeks of supply adds demand context to the inventory quantity.

Weeks of supply example

Assume a retailer has:

• On-hand inventory: 300 units
• Average weekly demand: 50 units

Weeks of Supply = 300 ÷ 50 = 6 weeks

If supplier lead time is eight weeks, six weeks of supply may indicate a replenishment problem. If lead time is two days and the product can be reordered easily, six weeks could represent unnecessary excess.

Should you use on-hand or inventory position?

Use the version that matches the decision being made. On-hand weeks of supply describes physical coverage today. For purchasing, it can also be useful to consider confirmed incoming stock and committed demand.

A simplified inventory position is:

On Hand + On Order − Committed Demand

Be explicit about which definition is being reported so teams do not compare unlike numbers.

Historical demand vs. forecast demand

Average historical demand is simple, but it can be misleading when sales are seasonal or changing quickly. In those cases:

Forward Weeks of Supply = Available Inventory ÷ Forecast Weekly Demand

More sophisticated planning can calculate coverage against the actual forecast for each future week rather than one flat average. See inventory forecasting methods.

Weeks of supply vs. days sales of inventory

Days Sales of Inventory is typically a financial metric calculated from average inventory value and COGS. Weeks of supply is often an operational SKU-level coverage metric based on units and expected demand.

They are related concepts but should not be treated as interchangeable calculations.

Weeks of supply vs. inventory turnover

Inventory turnover describes how often average inventory is sold and replaced during a period. Weeks of supply asks how long the current inventory is expected to last.

What is a good weeks-of-supply target?

There is no universal target. The appropriate coverage depends on supplier lead time, order frequency, demand variability, minimum order quantities, shelf life, stockout cost, and the ability to transfer stock between locations.

A product with a twelve-week supplier lead time needs a different policy from an item replenished locally every day.

Using weeks of supply for replenishment

Coverage can help teams prioritize purchase decisions. Products with coverage approaching their replenishment lead time deserve attention, especially when there is little safety stock or supplier performance is unreliable.

For a threshold-based approach, combine coverage with reorder points and inventory replenishment.

Weeks of supply across multiple locations

Calculate coverage by location. Ten weeks of company-wide stock can hide a store with one week remaining and another with nineteen. That imbalance may be solvable with a transfer instead of another supplier order.

Common mistakes

• Using stale average demand
• Ignoring seasonality or promotions
• Combining unlike locations
• Counting inbound stock twice
• Ignoring committed demand
• Using one target for every SKU
• Failing to account for shelf life

How Stash fits

Stash combines stock visibility, forecasting, purchasing, suppliers, and multi-location inventory. That context helps operators interpret inventory coverage as a purchasing decision rather than an isolated metric.

Frequently asked questions

How do you calculate weeks of supply?

Divide available inventory by average or forecast weekly demand, using consistent units.

Is more weeks of supply better?

No. Too little coverage increases stockout risk, while too much can tie up cash and increase carrying cost, spoilage, or obsolescence.

Should weeks of supply include purchase orders?

It can, but distinguish physical on-hand coverage from projected coverage that includes confirmed inbound inventory.