Weeks of Supply (WOS)

Weeks of Supply (also "weeks of stock", "weeks of cover", "weeks forward cover") is a retail inventory metric expressing how many weeks current stock will last at the current rate of sale. It is one of the core in-season health metrics in Merchandise Financial Planning (MFP), sitting alongside Sell-Through Rate, GMROI and Open-to-Buy (OTB), and it is the operational complement to Inventory Turnover.

Sourcing note: This was a web-only harvest — Reddit and YouTube returned no usable sources (see Gaps). Sourcing is vendor-knowledge-base dominated; treat the formula/usage as durable and the numeric benchmarks as volatile and vendor-derived.

How it works

WOS = current inventory ÷ average weekly sales — the count of weeks on-hand stock covers at the present sell rate (Wair; Cogsy). Worked example: 1,000 units in stock selling 200 units/week = 5 weeks of supply (Cogsy).

Two variants are distinguished by which demand they use (Flieber):

WOS (trailing) — calculated from historical demand (average past weekly sales).
Forward Weeks of Supply (FWOS) — calculated from forecasted future demand, estimating how long a SKU will stay in stock going forward. This ties WOS to Demand Forecasting.

How retailers use it

What sources report retailers do with WOS:

Set targets by item type — for replenishable basics, the WOS target is set within lead-time buffer plus safety stock; for seasonal items, WOS should track toward zero by the end of the selling period (Toolio). This links to Safety Stock Optimisation.
Trigger replenishment — a replenishment review fires immediately when FWOS falls below the reorder lead time (Toolio).
Trigger markdowns — WOS exceeding a category threshold is used as a markdown signal; e.g. a fashion category may trigger action if sell-through is below plan by week four and WOS exceeds the remaining selling window (Umbrex). See Markdown Optimisation.
Early dead-stock warning — monitored with Sell-Through Rate and GMROI, a lagging sell-through after three weeks combined with WOS climbing above target signals dead-stock risk (Wair).

The buying-side counterpart is Open-to-Buy (OTB) — the forward purchasing budget that factors in on-hand inventory and the target stock level needed to hit budgeted sales (Toolio).

Relationship to adjacent metrics

WOS / days-of-supply measure duration inventory will last (operational planning); Inventory Turnover measures frequency of selling and restocking (financial efficiency). Sources frame the two as complementary, not interchangeable (Unisco).
Days of inventory on hand and inventory turnover are inversely related: DSI = 365 ÷ inventory turnover ratio (Ware2Go). WOS is the weeks-denominated cousin of days-of-supply / DSI — see dangling Days of Inventory (DSI).

Benchmarks (as-of 2026-06-26)

No source stated a direct numeric "healthy WOS range" for apparel — vendors express WOS targets relationally (lead-time + safety stock; toward zero by season end) rather than as fixed weeks. The closest quantified anchors are adjacent metrics, which convert inversely to WOS (higher turns → lower WOS):

Metric	Reported range	Source	Note
Fashion inventory turns — luxury/high-end	~2–4× / year	StyleMatrix	undated, vendor — stale-risk
Fashion inventory turns — mid-market apparel	~4–8× / year	StyleMatrix	undated, vendor — stale-risk
Fashion inventory turns — fast fashion/value	~8–12× / year	StyleMatrix	undated, vendor — stale-risk
Apparel sell-through target	65–85% (fast-fashion 85%+; basics 65–70%)	Shopify (2026)	—
Seasonal/limited drop sell-through	>80% within launch window	Toolio	undated, vendor

What practitioners report (caveats & criticism)

Multi-SKU averaging trap — WOS becomes misleading the moment you look at more than one SKU: merging fast-movers (which stock out first) and slow-movers is poorly defined and can either hide or exaggerate lost-sales risk (Singuli).
Rounding error from the weekly denominator — a "4-week" item that is really 22 days of cover means over-purchasing by ~6 days, trapping cash (Cogsy).
Seasonality distortion — at peak, spiking weekly sales depress WOS even with stable stock; off-peak, WOS inflates and can mask overstocking (Orders in Seconds).
Garbage-in limits — WOS ignores marketing campaigns, DTC events and other demand drivers, so outputs are only as good as the inputs (Inventory Planner).

Most planning vendors (Wair, Toolio, Cogsy) treat WOS as a core inventory-health KPI [https://wair.ai/wos-meaning-and-how-to-calculate-it/]. Singuli argues it is misleading beyond a single SKU and that fill rate is the better service-level metric [https://singuli.co/blog/weeks-of-supply-formula/]. Note: Singuli sells fill-rate-based optimisation, so this is partly vendor positioning. Both sides agree on the formula itself.

Key terms

Term	Meaning
WOS	Current inventory ÷ average weekly sales (weeks of cover at current sell rate)
FWOS	Forward Weeks of Supply — same ratio using forecasted demand
DSI / Days of Supply	Days-denominated equivalent; DSI = 365 ÷ inventory turnover
Rate of sale	Average units sold per week, the WOS denominator
Fill rate	Singuli's proposed alternative service-level metric (Fill Rate)