Inventory management is the highest-leverage operational skill for Amazon FBA sellers. Run out of stock and your ranking drops, your ad campaigns destabilize, and it takes weeks to recover. Over-order and you drown in storage fees, aged inventory surcharges, and disposal costs.
For a deeper dive, see our inventory forecasting guide.After managing over $2M in FBA inventory, here's the exact restock system we use to maintain 98%+ in-stock rates while keeping storage costs under 3% of revenue.1. The Restock Formula
Order Quantity = (Avg Daily Sales x Lead Time Days + Safety Stock) - Current FBA Inventory - Inbound Units
Safety Stock = Avg Daily Sales x (Max Lead Time - Avg Lead Time) x 1.5
Example: You sell 10 units/day, supplier lead time is 20-30 days, you have 200 units in FBA and 150 inbound.
Safety Stock: 10 x (30 - 20) x 1.5 = 150 units
Order Qty: (10 x 30 + 150) - 200 - 150 = 100 units
This formula prevents stockouts during the worst-case supplier delay while avoiding excess inventory.
2. Lead Time: The Most Underestimated Variable
Most sellers calculate lead time as "supplier manufacturing time + shipping." The real lead time is longer:
- Manufacturing time: 15-25 days (often 30+ for first orders)
- Inland transport to port: 3-7 days
- Customs clearance: 3-10 days (longer for new products or restricted categories)
- Ocean freight: 20-35 days (China to US West Coast vs East Coast)
- Amazon receiving: 5-15 days (peak season can be 20+)
Total realistic lead time: 46-92 days. If you're only accounting for 30 days, you're already behind.
3. IPI Score: The Threshold That Matters
Amazon's Inventory Performance Index (IPI) targets 400+. Below 400 means storage limits and penalty fees.
Three strategies to boost IPI:
- Sell through slow movers: Products with turnover under 90 days are dragging you down.
- Reduce excess inventory: Units that have sat for 90+ days with zero sales in the last 30 days.
- Increase sell-through rate: Boost sales on existing inventory via PPC, coupons, or Lightning Deals.
4. The 90-Day Inventory Rule
Never hold more than 90 days of inventory for any single ASIN. Here's why:
Amazon charges long-term storage fees (LTSF) after 365 days. But the real cost is opportunity cost — the capital tied up in slow inventory could be earning 20-30% margin on a faster-moving product.
Implement a "90-day alerts" system: when any ASIN has 120+ days of cover, create a plan to reduce inventory. Options: coupon code, bundled offer, outlet deal, or removal.
5. Seasonal Inventory Tuning
- Pre-Q4 (September-November): Reduce safety stock by 20%. Amazon's receiving slows down.
- Post-Q4 (January-February): Aggressively reduce inventory from seasonal products.
- Prime Day preparation (June): Increase safety stock by 30% for top performers.
6. The Supplier Communication Protocol
Send a restock PO to your supplier with clear deadlines:
- Ready to ship by (date): Must leave factory by this date
- Late penalty clause: A 2% discount per week of delay
- Pre-shipment photos: Require photos of packed cartons before shipment release
This creates accountability. Without deadlines and consequences, suppliers prioritize bigger buyers over you.
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