What weeks of supply means in FBA
Weeks of supply (WOS) tells you how many weeks your current on-hand inventory will last if sales continue at their recent pace. It is the weekly-resolution cousin of days of supply and the metric most often used in executive-level inventory reviews and supplier planning conversations.
For Amazon FBA sellers, WOS is especially useful when communicating with suppliers who think in weekly or monthly production cycles. Telling your factory you need “10 weeks of cover” is more intuitive than saying “70 days of supply” when negotiating lead times and order quantities.
WOS also maps cleanly to Amazon’s own reporting cadences. The IPI dashboard evaluates inventory health on rolling windows that align with weekly snapshots, making WOS a natural fit for tracking whether you are over- or under-stocked relative to Amazon’s expectations.
Weeks of supply formula
Example: a $1.8M wholesale seller
A wholesale FBA seller running 45 SKUs at $1.8M annual revenue (average selling price $28). Take a top-performing SKU:
- On-hand FBA inventory: 2,100 units
- Units sold in last 8 weeks: 1,120
- Average weekly sales: 1,120 ÷ 8 = 140 units/week
Weeks of supply = 2,100 ÷ 140 = 15 weeks
This seller’s total lead time is 5 weeks (wholesale replenishment from a US distributor: 2 weeks order processing + 1 week ground shipping + 2 weeks FBA check-in). With 15 weeks of supply, they are carrying 10 weeks of buffer beyond their lead time. That is likely overstocked for a wholesale SKU with stable demand and short lead times.
WOS benchmarks for FBA:
| WOS Range | Status | Typical scenario |
|---|---|---|
| < Lead time in weeks | Stockout risk | Reorder immediately |
| Lead time + 2 to 4 weeks | Healthy | Good safety buffer |
| Lead time + 4 to 12 weeks | Heavy | Storage fees accumulating |
| > 26 weeks | Excess | Long-term storage fee territory |
FBA-specific considerations
Weeks of supply behaves differently in FBA than in a traditional warehouse because of how Amazon structures storage fees and capacity limits:
Storage fee cliffs make WOS thresholds concrete. Amazon charges monthly storage fees that jump significantly at the 6-month (26-week) and 12-month (52-week) marks with aged inventory surcharges. A SKU sitting at 25 WOS is fine; the same SKU at 27 WOS starts incurring higher per-unit fees. WOS maps directly to these fee boundaries.
Restock limits set a practical WOS ceiling. If Amazon allocates you 3,000 units of storage for a category and you sell 300 units/week, your maximum WOS is 10 weeks regardless of what you want it to be. Sellers with tight allocations need to accept lower WOS and compensate with more frequent, smaller shipments.
Weekly granularity hides day-level risk. A SKU at 2 WOS (14 days) with a 10-day lead time looks safe at weekly resolution but actually has only 4 days of buffer. For reorder decisions on fast-moving SKUs, convert WOS back to days of supply to see the real margin.
Common mistakes
- Mixing calendar weeks with selling weeks. If your product sells primarily Monday through Friday (B2B or office supplies), a 7-day week overstates your weekly velocity. Use actual selling-day velocity or stick with days of supply for accuracy.
- Using WOS for reorder triggers on long-lead-time SKUs. For products with 60+ day lead times, the rounding inherent in weekly measurement (each week = 7 days of precision loss) can cause you to reorder a week late. Use days of supply for the actual reorder trigger and WOS for reporting and planning conversations.
- Comparing WOS across SKUs without normalizing for lead time. A SKU with 8 WOS and a 4-week lead time has 4 weeks of buffer. A SKU with 8 WOS and a 10-week lead time is already 2 weeks past its reorder point. Always evaluate WOS relative to the SKU’s specific lead time.