Retail and ecommerce businesses need enough stock to meet customer demand without tying up more cash than necessary. Weeks of supply, or WOS, estimates how long available inventory may last at a given sales rate.
This metric can help inventory planners compare coverage across products and identify items that may require attention. However, it does not determine a purchase order on its own. Lead time, incoming stock, demand variability, supplier requirements, and service goals also affect when and how much inventory to buy.
This guide explains the weeks of supply formula, how to calculate WOS and forward WOS, and how to use the results alongside Days of Stock and replenishment timing.
Use sellable inventory rather than total on-hand stock if some units are reserved, damaged, or otherwise unavailable.
What Is Weeks of Supply in Inventory Planning?
Weeks of supply is an inventory coverage metric that estimates how many weeks available stock may last at the current or expected sales rate.
For example, a product with five weeks of supply is expected to support approximately five more weeks of sales if sales continue at a similar rate.
Retailers can use WOS to compare coverage across products, identify possible stockout risks, and flag products with more stock than projected demand requires. The result should still be considered alongside incoming purchase orders, replenishment time, demand variability, and purchasing constraints before an order is placed.
How to Calculate Weeks of Supply
To calculate weeks of supply, divide the inventory available for sale by the average number of units sold per week.
The result estimates how long the current stock may last if sales continue at the rate used in the calculation. Before calculating WOS, confirm that both the inventory quantity and sales period are defined consistently.
Weeks of Supply Formula
The standard weeks of supply formula is:
Weeks of Supply = Available Inventory ÷ Average Weekly Units Sold
In this calculation:
- Available inventory is the stock that can currently be sold.
- Average weekly units sold is the average number of units sold during the selected historical period.
Use a sales period that reflects typical demand. A period affected by major promotions, seasonal peaks or extended stockouts may produce a rate that is not representative of future sales.
Weeks of Supply Calculation Example
Suppose a retailer has 100 shirts available and sells an average of 20 shirts per week.
100 units ÷ 20 units per week = 5 weeks of supply
At that sales rate, the available inventory is expected to last approximately five weeks.
Five weeks of supply is not automatically too high or too low. The retailer must compare that result with factors such as supplier lead time, demand variability, incoming orders, and the amount of buffer stock needed to protect product availability.
How Stockouts Affect Rate of Sale
Stockouts can cause the sales rate used in the weeks of supply formula to understate demand.
Suppose a retailer sold 30 shirts during a 30-day period, but the product was out of stock for 15 of those days. Dividing 30 units by all 30 calendar days would produce an average of one unit per day.
However, the product sold 30 units during only 15 in-stock days. Based on the days when the item was available, the adjusted rate of sale would be two units per day.
This adjustment may provide a more useful estimate than treating stockout days as days with no demand. It still cannot show exactly how many customers would have purchased the item if it had remained available. Some customers may have selected another product, purchased elsewhere or abandoned the purchase.
What Is Forward Weeks of Supply?
Forward Weeks of Supply, or FWOS, estimates how long current inventory may last based on forecasted demand rather than a historical sales average.
A simple forward weeks of supply formula is:
Forward Weeks of Supply = Available Inventory ÷ Average Forecasted Weekly Sales
Suppose a retailer has 100 shirts available and an upcoming promotion is expected to increase sales to 25 units per week.
100 units ÷ 25 forecasted units per week = 4 forward weeks of supply
The available stock is forecast to last approximately four weeks during the promotion.
FWOS may provide a more relevant estimate when demand is expected to change because of seasonality, promotions, growth or declining product interest. Its reliability still depends on forecast quality. When forecast demand varies significantly from one week to the next, planners may need to compare inventory with each future week’s expected sales rather than relying on one weekly average.
Weeks of Supply vs. Days of Stock: What Is the Difference?
Weeks of Supply and Days of Stock express the same general idea: how long available inventory may support demand. The main difference is the time unit used.
The Days of Stock formula is:
Days of Stock = Available Inventory ÷ Average Daily Units Sold
For example, a retailer with 88 units available and average sales of four units per day has 22 days of stock. That is equivalent to approximately 3.1 weeks of supply.
A daily view can be useful when:
- Products sell quickly
- Orders are reviewed frequently
- Lead times are short
- Shelf life is limited
- A difference of several days could change a purchasing decision
Days of Stock provides more granular timing, but it is not automatically more accurate. Both calculations depend on reliable inventory, sales and forecast data.
Inventory Planner calculates Days of Stock at a daily level and combines it with forecast demand and replenishment data to generate purchasing recommendations across products. Retailers can use these recommendations alongside forecasts, replenishment timing and purchasing constraints to decide what and when to buy.
See how Inventory Planner can help you forecast demand and make more informed purchasing decisions. Book a demo.
How Replenishment Timing Affects Weeks of Supply
Weeks of supply becomes more useful when it is compared with the time required to replenish the product.
Lead time is the period between placing a purchase order and having the inventory available for sale. Depending on the business, it may include order approval, production, shipping, customs, and warehouse receiving.
The relationship between coverage and replenishment time can indicate how urgent the next purchasing decision may be:
- Coverage is longer than the replenishment time: The business may have time before another order is required. The decision still depends on the review schedule, safety buffer, and expected demand before the next delivery.
- Coverage is close to the replenishment time: The product may need to be reordered soon to reduce the risk of running out before the next shipment arrives.
- Coverage is shorter than the replenishment time: The product may face a risk of running out before the next order arrives unless an incoming order, transfer or expedited shipment can cover the difference.
Before placing an order, planners should also account for open purchase orders, minimum order quantities, case packs, supplier reliability, and any expected changes in demand. WOS supports the decision, but these additional inputs determine whether the order timing and quantity are practical.
How to Interpret a Weeks of Supply Calculation
A weeks of supply calculation does not have one universally good result. The appropriate level depends on the product, demand pattern, and replenishment conditions.
When interpreting WOS, consider:
- Replenishment time: Coverage may be too low if stock is expected to run out before more inventory can arrive.
- Demand variability: Products with unpredictable sales may require a larger buffer than products with stable demand.
- Supplier reliability: Delays or inconsistent delivery times increase the risk associated with low coverage.
- Ordering frequency: Businesses that review and place orders frequently may be able to operate with less coverage.
- Product lifecycle: Seasonal, perishable, or trend-driven products may carry a higher cost when inventory remains unsold.
- Product-availability goals: A business that prioritizes high product availability may accept more inventory than one focused primarily on reducing working capital.
Targets should therefore differ across product groups rather than applying one WOS benchmark to every SKU. Minimum order quantities, case packs, storage capacity, and incoming purchase orders may also affect what the business can order in practice.
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Weeks of Supply Formula FAQs
Is Weeks of Supply the Same as Weeks of Cover?
Yes. Weeks of supply and weeks of cover generally describe the same inventory coverage metric: how many weeks available stock may last based on current or forecasted demand. However, businesses may define the inputs differently, so confirm how inventory and sales are measured.
Should Incoming Purchase Orders Be Included in Weeks of Supply?
The standard weeks of supply formula normally uses inventory that is currently available to sell. Incoming purchase orders can be tracked separately to estimate future coverage after those units arrive.
Keeping the two figures separate helps planners distinguish the stock available now from inventory that may be delayed, received only partially or unavailable before the current stock runs out.
How Often Should Weeks of Supply Be Recalculated?
Weeks of supply should be updated often enough to reflect meaningful changes in sales, inventory, and incoming orders. A fast-moving or seasonal product may require frequent review, while a stable, slow-moving item may need less frequent updates.
The appropriate cadence depends on sales velocity, purchasing frequency, and how quickly the business can respond to a change in coverage.
Can Weeks of Supply Be Zero or Negative?
Weeks of supply is zero when no sellable inventory remains. It should not normally be negative because available inventory cannot be less than zero for the purposes of the formula.
If average weekly sales are zero, the formula does not produce a useful finite result. The product should instead be reviewed for insufficient sales history, inactive demand, or an inventory record issue.
What Is the Difference Between Weeks of Supply and Weeks on Hand?
Weeks of supply and weeks on hand are sometimes used interchangeably. Some retailers use weeks on hand for coverage based on current inventory and historical sales, while weeks of supply may refer to coverage based on current or forecasted demand. Because definitions vary across businesses and software platforms, confirm which inventory and sales inputs are included.