Amazon inventory planning sits at the intersection of two expensive problems: run out of stock and you lose Buy Box eligibility, ranking momentum, and revenue while you wait for replenishment. Overstock and you accumulate storage fees, risk long-term storage penalties, and tie up capital in units that aren't selling.

Most FBA sellers manage inventory reactively — reordering when stock gets low, guessing at lead times, hoping the velocity holds. That approach works until it doesn't, and when it breaks, it breaks at the worst time: peak season, during a promotional push, or on a product that's gaining momentum.

The variables that determine reorder timing

An inventory plan needs to account for four variables that interact:

Sales velocity. Your average daily or weekly units sold. This should be segmented — recent velocity (last 14 days) vs. rolling average (last 90 days) will diverge during trends, seasonal peaks, and after promotions. Planning to one and ignoring the other creates blind spots.

Lead time. The time between when you place a purchase order and when inventory arrives at an Amazon fulfillment center — including production time if you're manufacturing, transit time, your own receiving and prep time, and the time Amazon takes to receive and process the inbound shipment. Every step introduces variability, and that variability has to be built into your buffer.

Minimum order quantity. Your supplier's MOQ determines how much you can order at a time, which affects how often you're placing orders and how much capital is tied up per replenishment cycle.

Cover days target. How many days of inventory do you want on hand as a floor? This is a business decision, not a math problem — it depends on your cash position, your supplier reliability, your category's seasonality, and your tolerance for stockout risk.

The reorder point formula

The basic reorder point formula is:

Reorder Point = (Daily Sales Velocity × Lead Time in Days) + Safety Stock

Safety stock is the buffer above the lead time demand that protects against velocity spikes and lead time variability. A common approach is to use 50% of your lead time demand as safety stock — meaning if you sell 10 units per day and your lead time is 30 days, your reorder point is (10 × 30) + (10 × 30 × 0.5) = 300 + 150 = 450 units.

That's the starting point. The adjustments come from knowing your specific situation:

Seasonality is the variable most sellers underweight

A seller who plans inventory based on 90-day rolling velocity in August is planning based on slow-season numbers for a Q4-heavy product. By the time October demand accelerates, it's too late to replenish without airfreighting or accepting a stockout. Build seasonal demand curves into your model, not just trailing averages.

The real cost of a stockout

Most sellers calculate stockout cost as lost revenue during the stockout period. The actual cost is higher because of what happens to ranking and Buy Box after you return to stock.

Amazon's ranking algorithm weights recent sales velocity heavily. A product that goes out of stock loses its sales history during that period, which depresses the ranking signal when it returns. Depending on category competitiveness and how long the stockout lasted, recovery to pre-stockout rank can take days to weeks — meaning the revenue loss extends well beyond the stockout window.

For a product with strong organic ranking, this extended recovery cost can be 3–5× the direct revenue loss during the stockout. It's one of the reasons that the sellers who grow fastest treat inventory availability as a strategic investment, not just a cost to minimize.

The overstock problem: long-term storage and capital efficiency

Overstocking is the opposite error, and it has its own compounding costs. Amazon charges monthly storage fees per cubic foot, but the real penalty is long-term storage fees that trigger at 365 days: $6.90 per cubic foot or $0.15 per unit, whichever is greater.

For low-velocity products with high cubic volume — bulky items, kits, large-format goods — long-term storage fees can exceed the product's margin. Sellers who don't track aging inventory find out about long-term storage fees when they hit their settlement reports.

The discipline is monitoring inventory age by ASIN and taking action before the 365-day trigger: promotional pricing to clear slow-movers, removal orders to pull units you'd rather sell through other channels, or disposal orders for units whose FBA value is below removal cost.

What to look for in FBA inventory tools

The right inventory tool for FBA needs to do three things well: pull real-time inventory and sales data via SP-API (not exports or scrapes), calculate reorder points with configurable lead times and safety stock, and alert you when action is needed — not just show you charts.

Tools that show you inventory data without telling you what to do with it create dashboard fatigue. The goal is a system that tells you: reorder this ASIN today, you have 18 days of stock remaining at current velocity. That's an actionable output. A graph of inventory levels over time is not.

TenEightOne OP's Inventory module connects directly to the SP-API and provides reorder intelligence with configurable parameters — not just what's in stock, but what needs attention and when. Learn more →