Weeks of Supply (WOS) Formula
How apparel planners calculate weeks of supply, why WOS drives every replenishment and markdown decision, and the benchmark ranges by channel.
What Weeks of Supply measures
Weeks of supply is the number of weeks of inventory remaining at current sales velocity. It is the primary signal for replenishment, reallocation, and markdown timing.
WOS = On-Hand Units ÷ Average Weekly Unit SalesWOS uses trailing average weekly unit sales (typically 4 or 6 weeks). The forward-looking version is FWOS — use that when the sales curve is about to change.
Worked apparel example
A DTC brand has 520 units on hand of a core tee. Over the last four weeks, it has sold an average of 40 units per week.
WOS = 520 ÷ 40 = 13.0 weeks
At 13 weeks, this style is running heavy — the season has 6 weeks left. Without either a chase that cannot happen at this lead time, a markdown, or a reallocation to faster doors, the inventory will age out.
13.0 weeks of supply is heavy. Likely candidate for markdown or reallocation to faster doors.
Benchmark ranges
The "bad / good / great" framing works in a stable demand window. In a demand ramp (holiday, product launch), a high WOS is healthy. In a demand decline (post-season), a low WOS can still mean the residual aged out.
Failure modes we see
WOS calculated from the wrong velocity window. A 12-week trailing average on a 6-week selling season includes 6 weeks of zero — suppressing velocity and overstating WOS. Planners hesitate to mark down and the end-of-season dead stock balloons.
Specific patterns:
- Single-door WOS. Chain-level WOS hides door-level imbalances. 8 weeks chain is often 3 weeks at fast doors, 14+ at slow doors.
- Size imbalance buried. A style at 10 WOS overall can be 4 weeks on core sizes (stocking out) and 16 weeks on extended sizes (dead).
- No FWOS alternative. WOS alone misleads around forecast inflection points — pre-peak, post-peak, and new-launch windows.
How RetailNorthstar handles WOS
WOS runs at style × color × size × door × week, with a trailing window setting appropriate to each department's season length. Dashboards surface not just the WOS number but the action class — chase candidate, reallocation candidate, markdown candidate — based on WOS plus sell-through plus remaining season.
A single WOS number never triggers an action by itself. It is the combination of WOS + full-price sell-through + remaining season + lead time that identifies whether a style is a chase, a reallocation, or a markdown. Connected planning systems model all four together.
Related formulas
- Forward Weeks of Supply — the forward-looking variant for demand inflection points
- Inventory Turns — the full-period expression of WOS
- Sell-Through Rate — the other half of the chase/markdown decision
See how RetailNorthstar surfaces WOS at style × size × door, tied to the action it implies — chase, reallocate, or mark down.
Book a Demo →Frequently asked about Weeks of Supply (WOS)
What is the WOS formula?
WOS = On-Hand Units ÷ Average Weekly Unit Sales. The typical trailing window is 4 or 6 weeks. The result is how many weeks of selling the current inventory supports at current velocity.
When should I use WOS vs Forward WOS (FWOS)?
Use WOS (trailing average) in stable mid-season windows. Use FWOS (forecasted) at demand inflection points — pre-peak ramps, post-peak declines, and new launches — where trailing velocity will misrepresent forward demand.
What is a healthy WOS for apparel?
DTC: under 6 weeks is lean/great, 6–12 weeks is normal, 14+ weeks is heavy. Wholesale runs 2 weeks higher. Luxury runs 4+ weeks higher because carry-over is part of the model. Category inside channel matters too.
How does WOS drive reallocation decisions?
A style at 3 WOS at some doors and 15 WOS at others signals imbalance — reallocate from slow to fast doors before the slow inventory ages into markdown. Chain-average WOS masks this signal; door-level WOS surfaces it.