Amazon Management · Inventory Strategy
Amazon’s Inventory Performance Index, its low-inventory-level fee, and its aged-inventory surcharge now bracket sellers into a narrower operating window than ever. Miss on either side — too little stock or too much — and it shows up directly in fees, Buy Box eligibility, and lost sales.
THE 2026 FBA INVENTORY CORRIDOR
Quick Answer
Prevent stockouts by calculating a real reorder point per SKU — average daily sales × total lead time, plus safety stock — instead of a gut-feel buffer. Use Amazon’s Days of Supply and Low Stock flags weekly, size lead time on your 80th-percentile shipment history rather than the average, and keep 35–55 days of supply as your working target so you stay clear of both the low-inventory fee and the aged-inventory surcharge.
A stockout on a high-velocity ASIN does more than pause revenue for a few days. It can hand your Buy Box share to a competitor, reset organic ranking momentum built up through weeks of advertising spend, and drag down your Inventory Performance Index at the exact moment you need storage capacity to restock. Recovering from a single bad stockout on a best-seller often costs more in lost ranking and ad spend than an entire year of disciplined inventory planning would have cost.
Amazon’s IPI score runs from 0 to 1,000, and the minimum threshold sellers need to stay above is 400. Drop below it and Amazon can apply FBA storage capacity limits — which, in a cruel irony, makes it harder to send in the very replenishment shipment that would fix the stockout causing the low score in the first place.
| IPI Factor | What It Measures | Stockout Connection |
|---|---|---|
| In-stock rate | How often your ASINs were in stock over the trailing 30 days, weighted by sales velocity | Hit directly and fastest by stockouts on your best sellers |
| Sell-through rate | Units sold over 90 days relative to average units on hand | Overordering to avoid a stockout can tank this if velocity doesn’t keep up |
| Excess inventory | Share of stock that’s overstocked relative to forecasted demand | The overcorrection sellers make right after a stockout scare |
| Stranded inventory | Percentage of FBA stock with no active, sellable listing | Stock sitting stranded is functionally a stockout you’re already paying to store |
Most stockouts don’t come from bad luck — they come from reordering off a gut feeling instead of a number. The standard formula:
Safety stock absorbs the gap between your average scenario and your worst realistic one:
Worked example: a SKU selling 34 units/day on average, with a total lead time of 28 days and a 7-day safety buffer, gives a reorder point of (34 × 28) + (34 × 7) = 1,190 units. Once available FBA inventory drops to that level, it’s time to place the next order — not when the shelf looks low.
Seller Central’s Restock Inventory tool (under Inventory → Inventory Planning) generates replenishment quantity and ship-date suggestions based on your sales history, lead time settings, and case pack quantities. It also flags listings as “Low Stock” once their days of supply drops below the lead time you’ve configured, signaling that a shipment may need to be expedited to avoid running out.
These native tools are a solid starting point but tend to miss two things: they don’t always separate supplier lead time from Amazon’s own receiving delays, and they can lag behind sudden demand spikes from a Lightning Deal or a competitor’s stockout. Sellers managing dozens of SKUs typically pair Amazon’s tools with a dedicated forecasting spreadsheet or third-party inventory platform for that reason.
| Fee | Trigger | Why It Exists |
|---|---|---|
| Low-inventory-level fee | Historical days of supply falls below 28 days, measured per seller-FNSKU | Thin stock limits Amazon’s ability to distribute inventory across its fulfillment network |
| Aged-inventory surcharge | Units stored 181+ days | Discourages tying up fulfillment center space with slow-moving stock |
The practical takeaway: padding safety stock to feel safer against stockouts can push slow-moving units straight into aged-inventory territory. Both fees need to be modeled together, not treated as separate problems.
Replace any “reorder when it feels low” habit with the formula above, run individually for every SKU. Products with different velocities and lead times need different trigger points — a single shop-wide rule of thumb almost always under-protects your best sellers.
Pull your last six to eight inbound shipments and note the actual time from shipment creation to “Available” status. Use the 80th percentile of that range, not the average, so the majority of shipments land inside your planning window instead of half of them running late.
Size your buffer against the worst realistic combination of a demand spike and a lead-time delay, not just one or the other. This produces a buffer that actually holds up during a bad month instead of one calibrated to an easy one.
Amazon’s Days of Supply metric is a lagging indicator — it only fully adjusts after orders finish processing. A weekly check, rather than monthly, catches drift early enough to expedite a shipment before it becomes a stockout.
A simple trailing 30-day average misses the swings caused by Lightning Deals, PPC budget changes, and competitor stockouts that briefly redirect demand your way. Weighting recent sales more heavily catches these shifts faster than a flat monthly look-back.
Restock limits and IPI-driven capacity restrictions can cap how much of a reorder actually makes it into FBA at once. Holding a portion of safety stock in Amazon Warehousing & Distribution or a third-party warehouse gives you a release valve when FBA capacity tightens.
Amazon’s fulfillment center receiving time, typically five to seven business days outside peak periods, can stretch to two or three weeks during Q4 and Prime Day prep. Add that buffer directly into your lead time component rather than treating it as a surprise each year.
Reordering by gut feeling. Without a calculated reorder point, restocking decisions drift toward “whenever it feels low” — which is exactly how stockouts sneak up on fast movers.
Using average lead time instead of a percentile. Averages hide the slow shipments that actually cause stockouts; plan around the 80th percentile instead.
Overcorrecting after a scare. A stockout often triggers a bulk overorder, which trades a stockout risk for an aged-inventory and excess-inventory problem a few months later.
Ignoring FBA receiving time. Treating “shipment created” as “inventory available” understates lead time, especially heading into Q4.
Reviewing inventory monthly instead of weekly. Amazon’s Days of Supply metric lags; monthly reviews catch problems well after they’ve started.
Treating every SKU the same. A single shop-wide reorder rule ignores that different products have different velocity, lead time, and demand variability.
Weekly cadence
Check IPI and Days of Supply every week — Amazon recalculates IPI weekly using a rolling 13-week window.
Model both fees together
Plan reorder points against the 28-day and 181-day thresholds at the same time, not as separate exercises.
Fix stranded stock first
Clearing stranded inventory is usually the fastest IPI improvement available, and it’s stock you’re already paying to store.
Document your process
A written reorder process gets more accurate over time and survives a team member being out during a critical restock week.
Start with reorder points on your top three to five SKUs by revenue before building out a full system — that’s where a stockout does the most damage.
Overseas lead times make the 80th-percentile lead-time method essential; a single missed customs delay can wipe out a thin buffer.
Sellers also running Etsy, eBay, or Walmart storefronts need separate reorder logic per channel, since fulfillment lead times and demand patterns differ by platform.
High Dreams LLC is a Colorado-based digital growth agency that has shipped work for 150+ clients worldwide across Amazon, Walmart, Etsy, and eBay. The team includes a dedicated Amazon manager who tracks inventory metrics, order defect rate, and late shipment rate on a weekly basis — the same discipline this guide walks through — to catch restock and Buy Box risk before it compounds.
A reorder point is the inventory level that should trigger a new purchase order. The base formula is average daily sales multiplied by total lead time, plus safety stock. For Amazon FBA, lead time must include supplier production, freight, and Amazon’s receiving time, which can stretch to two or three weeks during Q4.
Amazon’s minimum IPI threshold is 400 out of a possible 1,000. Falling below it can trigger FBA storage capacity limits, so most sellers aim to stay comfortably above that line rather than treating 400 as a safe target.
Amazon’s low-inventory-level fee applies once your historical days of supply drops below 28 days at the individual seller-FNSKU level. Most sellers target 35 to 55 days of supply as a working baseline.
The reorder point is the trigger level that tells you when to place a new order. Safety stock is the buffer built into that trigger to absorb demand spikes or supplier delays.
The aged-inventory surcharge now starts at 181 days in storage, roughly 90 days earlier than the older long-term storage threshold. Pushing safety stock too high to avoid stockouts can walk slow-moving units straight into that surcharge, so both ends need to be modeled together.
Check your Inventory Performance Index and Days of Supply weekly rather than monthly. A weekly cadence catches demand spikes and lead-time slippage early enough to act before a stockout or a low-inventory fee actually hits.
Get a free FBA inventory audit from a team that tracks restock timing and account health for Amazon sellers every week.
Sources: Amazon Seller Central Restock Inventory and Inventory Performance Index documentation; industry reporting on 2026 FBA fee changes including the low-inventory-level fee and aged-inventory surcharge thresholds.
