Weeks of supply explained: the cover metric behind buy and reorder decisions

What is weeks of supply and how is it calculated?

Weeks of supply is an inventory coverage metric that estimates how many weeks available stock may last at the current or expected sales rate. It measures how long your current inventory will last at your current or forecasted rate of sale, and it is the foundation of sound replenishment decisions.

The standard weeks of supply formula is: weeks of supply equals available inventory divided by average weekly units sold. The key difference between weeks of supply and days sales of inventory (DSI) is that weeks of supply uses forecast demand, whilst DSI uses historical cost of goods sold. Weeks of supply is for operating planning, covering the next week or next four weeks, whilst DSI is for financial reporting and external comparison.

Buyers and planners run weeks of supply daily at the SKU level. It provides a cross-category standard to compare inventory coverage across categories regardless of volume differences.

How does forward weeks of supply differ from basic 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. Forward weeks of supply is the more accurate and actionable metric for planning because it accounts for the seasonality and trend shifts that trailing averages miss.

A simple forward weeks of supply formula is: forward weeks of supply equals available inventory divided by average forecasted weekly sales. For example, suppose a retailer has 100 shirts available and an upcoming promotion is expected to increase sales to 25 units per week. The available stock is forecast to last approximately four weeks during the promotion.

The tradeoff is that forward weeks of supply requires a forward-looking demand forecast, which adds complexity. Buyers should use four to eight recent weeks as their baseline sell rate and adjust for seasonality where it exists, so that a style that normally peaks in week 40 is evaluated against what its expected sell rate at that time of year is, not its all-time average.

What are typical weeks of supply targets for fashion categories?

Weeks of supply benchmarks vary substantially by category and business model. Apparel specialty retailers typically plan 10 to 14 weeks of supply, with weeks of supply ramping down post-peak in the season. Footwear requires 12 to 18 weeks, because size matrices force deeper inventory carry per style.

Fast fashion retailers may run on a lean four to six weeks of cover. Premium footwear may target 10 to 14 weeks due to unique size and style requirements. Basics like socks or vests might allow only three to four weeks' cover, reflecting consistent demand.

If an organisation targets four inventory turns per year, its implied weeks of supply target is 13 weeks. If it is running at eight turns, the implied weeks of supply is 6.5 weeks. Public apparel companies held inventory equal to 19.3 per cent of sales, implying sales-basis turns of 5.2 and days in inventory of 70, according to industry data from January 2026.

How should weeks of supply change across a season?

Plan-level weeks of supply is set per category, per week of season, and rolls daily on actual sell-through and inbound receipts. An apparel buyer planning a denim style for autumn might set plan weeks of supply at season start at 14 weeks, by week eight at 10 weeks, by week 12 at six weeks, and at end of season at two weeks to be cleared by markdown.

Weeks of cover calculations must change with the season. The summer season for sandals may see six weeks sufficing, whereas heavy jackets in winter may need a longer lead, supporting up to 12 weeks. Weeks of cover equals current on-hand stock divided by average weekly sales. If your weeks of cover is higher than the remaining weeks in the season, you have an overstock problem.

How do lead time and supplier constraints affect weeks of supply targets?

Your weeks of supply should always equal or exceed your supplier lead time, plus a safety buffer. If your supplier lead time is eight weeks, your minimum weeks of supply before placing a purchase order is not eight weeks but eight weeks plus a buffer for demand variability and supply uncertainty. A reasonable starting rule is: minimum weeks of supply target equals lead time plus lead time multiplied by 25 per cent safety buffer. For an eight-week lead time, that is approximately 10 weeks.

For a style with a 10-week lead time and moderate demand variability, a target range of 12 to 16 weeks of supply is defensible: 10 weeks to receive the order plus two to six weeks of buffer. When current weeks of supply drops below 12, a purchase order should be placed. Weeks of cover tells you how long until you are out. Lead time tells you how long until new stock arrives. If you have 10 weeks' cover and a 12-week lead time, you are already late.

Which retail planning systems support weeks of supply calculation?

Most modern merchandise planning platforms track weeks of supply as a core metric. Blue Yonder Merchandise Planning covers merchandise financial planning, assortment planning, allocation, and replenishment within a unified environment and is well suited to large multi-channel retailers with significant store and SKU complexity. Oracle Retail offers a comprehensive suite covering merchandise planning, assortment, allocation, pricing, and store operations, and is designed for large global retailers with complex, multi-tier planning requirements.

RELEX Solutions is an AI-native platform for retail and supply chain planning, with floor planning and space management sitting inside a much larger unified planning system. Its strength is connection: space decisions do not live in a silo; they flow into and out of demand planning, replenishment, pricing, and promotions. Infor is a cloud-based retail planning software designed to support businesses in optimising their operations and improving customer experiences, and it caters to retailers across various sectors, providing tools for merchandising, supply chain management, and customer engagement.

Board supports merchandise financial planning, assortment planning, and financial alignment in one platform and is best suited to fashion, apparel, and specialty retail, where financial, merchandising, and supply decisions must be aligned continuously.