What days of supply means in FBA
Days of supply (DOS) answers the simplest inventory question: how long until I run out? For Amazon FBA sellers, it is the primary metric for reorder timing. Every replenishment decision starts with knowing how many days of runway you have left for each SKU.
Unlike weeks of supply, which rounds to weekly increments, days of supply gives you daily precision. That precision matters when your lead time is 60 days and the difference between 62 days of coverage and 55 days of coverage is the difference between staying in stock and losing your BSR.
Most FBA operators target days of supply equal to their total lead time plus a safety stock buffer. If your supplier takes 45 days to produce and Amazon takes 14 days to receive and check in, your minimum comfortable DOS is roughly 59 days, plus whatever safety buffer your demand variability requires.
Days of supply formula
Example: a $2.4M private label seller
A private label seller running 22 SKUs at $2.4M annual revenue (average selling price $38, average daily velocity across the catalog of roughly 173 units/day). Take one mid-tier SKU:
- On-hand FBA inventory: 1,240 units
- Units sold in last 30 days: 420
- Average daily sales: 420 ÷ 30 = 14 units/day
Days of supply = 1,240 ÷ 14 = 88.6 days
This seller’s total lead time is 72 days (45-day production + 12-day ocean freight + 15-day FBA check-in). With 88.6 days of supply, they have roughly 16.6 days of buffer beyond their lead time. That is a reasonable safety stock cushion for a SKU with moderate demand variability.
What “good” looks like for FBA:
| DOS Range | Status | Action |
|---|---|---|
| < Lead time | Stockout risk | Reorder immediately or air-freight |
| Lead time to lead time + 30 | Healthy | Place next PO on schedule |
| Lead time + 30 to 180 | Overstocked | Delay next PO, consider promotions |
| > 180 | Excess | Liquidate or create removal order |
FBA-specific considerations
Days of supply in FBA is more complex than in traditional retail because you do not control the warehouse. Three FBA-specific factors change the math:
Inbound delays inflate your effective lead time. Amazon’s check-in process can add 7 to 21 days on top of your shipping time. Your DOS calculation needs to account for total lead time including check-in, not just the time until your shipment arrives at the FC.
Restock limits cap your ceiling. Even if you want 120 days of supply for safety, your storage allocation may only allow 60 days. This forces you to ship more frequently in smaller quantities, which raises per-unit logistics costs.
Seasonal velocity swings compress your window. A SKU selling 14 units/day in March might sell 35 units/day during Prime Day. If you calculate DOS using trailing 30-day averages, your number will look fine right up until demand doubles and your 88-day cushion becomes a 35-day cushion overnight. Use forward-looking forecasted demand during peak periods.
Common mistakes
- Using total inventory instead of fulfillable inventory. Reserved units (customer orders, FC transfers) and unfulfillable units are not available to sell. Including them inflates your DOS and gives you a false sense of security. Use only the fulfillable quantity from your Manage Inventory report.
- Ignoring inbound shipments in the calculation. If you have 800 units on hand and 1,200 inbound, your effective coverage is higher than the raw DOS suggests. But inbound units have uncertain arrival dates. Calculate DOS on fulfillable units only, then track inbound separately as a coverage extension with a date range.
- Using a lookback window that is too short or too long. A 7-day average is noisy and overreacts to a single promotional spike. A 180-day average smooths out real trend changes. For most FBA SKUs, a 30-day trailing average balances responsiveness with stability.