Managing FBA inventory sounds straightforward until you’ve watched an ASIN go out of stock on a Friday afternoon with a weekend of traffic on the way, or found yourself paying monthly storage fees on 400 units that aren’t moving. Both situations are common, and both come down to misreading the same set of signals in Seller Central.
This guide covers the key FBA inventory metrics, where sellers most often get them wrong, and how to build a system that catches problems before they cost you money.
Seller Central surfaces a lot of data, and most of it gets ignored. These four metrics are the ones that directly affect your costs and your sales velocity.
FBA Available Units. This is your sellable inventory count at Amazon’s fulfilment centres — what customers can actually buy right now. The number that matters is not what you shipped in; it’s what Amazon considers available after accounting for units in transit, reserved, or flagged.
Inventory Performance Index (IPI). Amazon’s single-number score for how well you’re managing your FBA inventory. Scores above 450 are considered healthy; dropping below 400 can trigger storage limits. The IPI is driven by four sub-metrics: excess inventory percentage, stranded inventory rate, in-stock rate, and sell-through rate.
Stranded inventory. Units sitting at Amazon’s warehouse that have no active listing associated with them — they can’t be sold but you’re still paying storage. According to Amazon’s Seller Central documentation, sellers are responsible for resolving stranded status within 30 days or Amazon may initiate automated removals.
Aged inventory. Units that have been in FBA for 181-270 days (medium-aged) or 271-365 days attract surcharge fees. If you have an ASIN that’s been sitting for six months, Amazon is actively charging you to store it.
Relying on units-shipped as a reorder signal. The right reorder trigger is available units relative to your sell-through rate — not total shipped. A common mistake is sending in large quantities and using that as the mental anchor for how much stock you have, when what matters is how fast the available count is falling.
Ignoring stranded inventory until it’s a problem. Stranded units don’t generate revenue and don’t reduce your storage fees — they just sit there accumulating cost. A suppressed listing that creates stranded units can go unnoticed for weeks, by which time you’ve lost sales and incurred unnecessary storage charges simultaneously.
Setting reorder points without accounting for lead time. The standard transit time from a supplier or 3PL to an Amazon fulfilment centre, plus check-in and receiving time, can run 10-21 days. If your reorder point only accounts for 5 days of stock cover, you’re regularly risking a stockout window.
Overordering to avoid stockouts. The response to stockout fear is often overstocking — which generates aged inventory fees and IPI drag that then create a different set of problems. The goal is a reorder point that reflects your actual velocity, not a comfortable buffer that accumulates for months.
Sell-through rate is the ratio of units sold over 90 days to the average number of units on hand over the same period. Amazon uses it as part of your IPI calculation.
A sell-through rate below 2 signals that your inventory is moving slowly relative to how much you’re holding. The fix isn’t always to lower price — it can also mean you’re overstocked for the current velocity, and a removal order for excess units would both reduce fees and improve your IPI.
Jungle Scout’s annual seller data consistently shows that FBA sellers who actively manage sell-through rate see lower storage fee expenses — not because they hold less inventory in absolute terms, but because the ratio of stock to velocity is healthier.
The direct cost is obvious — you can’t sell units you don’t have. The less visible cost is what happens to your organic ranking while you’re out of stock.
Amazon’s algorithm uses recent sales velocity as a ranking signal. A listing that goes out of stock for 3-5 days experiences a ranking drop that can take weeks to recover. Helium 10’s research on listing velocity shows that the BSR impact of a stockout compounds over the first 48 hours — not linearly but accelerating.
For sellers with strong seasonal ASINs, a weekend stockout during peak traffic can knock you off page one right when buyers are most active.
You don’t need expensive software to manage FBA inventory well. A basic system covers three things.
1. A reorder point that reflects actual lead time. Calculate your average daily sales velocity (units sold in the last 30 days divided by 30), multiply by your supplier-to-FBA lead time in days, and add a buffer of 10-15 days. This is your reorder trigger.
2. A weekly stranded inventory check. Set a 15-minute calendar block each week to review your stranded inventory in Manage Inventory. Most stranded issues have a straightforward fix — relist, re-enable, resolve the suppression flag — if caught within a few days.
3. Real-time stockout alerts. Seller Central’s restock reports are backward-looking. SentryKit’s Out of Stock alert fires as soon as your available unit count hits zero, so you can respond immediately rather than discovering the issue on a Monday morning review.
For a broader look at inventory alerts and how to use them to prevent FBA cost creep, see our guide on Amazon Inventory Alerts and FBA cost control.
The Inventory Performance Index is a composite score Amazon uses to summarise your FBA inventory health. Amazon updates it weekly and makes it visible in the Inventory > Inventory Planning dashboard in Seller Central.
The four components that feed it:
Sell-through rate. Units sold in the past 90 days divided by average units on hand. Amazon wants this high — it signals you’re not sitting on dead stock. The target threshold Amazon considers healthy is above 2.0 on most categories.
Excess inventory percentage. Units Amazon considers overstocked relative to your expected 90-day sell-through. Units above a certain days-of-supply threshold get flagged as excess. These drag your IPI down and attract long-term storage fees.
Stranded inventory rate. The percentage of your FBA inventory that has no active listing. Even a small amount of stranded stock has an outsized effect on IPI if left unresolved. A weekly stranded inventory review is the single fastest IPI fix for most sellers.
In-stock rate. How often your ASINs are in stock across the trailing 30 days, weighted by sales velocity. Stockouts on high-velocity ASINs tank this metric faster than slow movers. Amazon gives more weight to ASINs that were recently selling well — a stockout on a seasonal peak item affects IPI significantly.
Improving IPI almost always comes down to two actions: clearing stranded inventory and tightening reorder cycles to reduce both stockouts and overstock. The other two components (sell-through and excess) improve as a consequence of those two actions.
Sellers with IPI scores below 400 at the end of a measurement period face FBA storage capacity limits. Amazon publishes the exact thresholds in Seller Central, but the mechanism is: low IPI → reduced storage capacity → inability to send in replenishment → potential stockout on high-velocity ASINs.
Amazon shifted from the older storage type limits (standard-size, oversize) to a cubic-feet-based capacity model in 2023, which it has iterated on since. Capacity is now allocated monthly, and confirmed limits appear in your Shipping Queue.
The most common way sellers hit limits: overordering during Q4 preparation, then carrying excess inventory into Q1 when velocity drops and storage fees are at their highest (January-September rates vs. October-December rates). Units that don’t sell through by January attract the long-term storage fee surcharge on top of the standard monthly rate.
If you’re approaching your limit, the options are: create removal orders for excess or aged units, liquidate via Amazon’s liquidation programme, or run a targeted price reduction to accelerate sell-through before the measurement date. For a breakdown of the IPI measurement windows, check Amazon’s Seller Central documentation on storage limits.
Q4 creates more FBA inventory problems than any other period — not because of the peak itself, but because of what happens after it. FBA sellers who understand how Amazon uses seasonal badges like the Arrives Before Dec 25 badge can also use them to drive velocity before the holiday cutoff.
The pattern: sellers over-prepare for Q4, carry units that don’t sell through, enter January with high inventory levels, face peak storage fee surcharges, and simultaneously watch their sell-through rate drop as velocity normalises. The result is an IPI hit precisely when they’re trying to plan for Q1.
Planning approach that avoids this:
Set a Q4 order ceiling based on 120% of last year’s Q4 velocity, not on optimistic projections. The upside of slightly under-ordering (a short stockout late in Q4) is usually smaller than the downside of carrying 30 days of excess inventory into January storage fees.
Plan a removal order trigger date. Decide before Q4 starts that any unit with more than 60 days of supply on January 1st gets a removal order. This is a pre-committed decision, not a reactive one — it prevents the common mistake of holding out for a price recovery that rarely comes in January.
Use real-time stock level visibility. Going out of stock during the last two weeks of November or first two weeks of December is particularly costly given advertising rates and organic rank. SentryKit’s Out of Stock alert gives you the earliest possible signal to push an emergency replenishment or adjust price to preserve rank while units are in transit.
FBA inventory management is the process of tracking, replenishing, and optimising the stock you hold at Amazon’s fulfilment centres — including monitoring available units, sell-through rate, stranded inventory, and aged inventory to avoid stockouts and excess storage fees.
Amazon considers an IPI score above 450 healthy. Scores below 400 can trigger storage capacity limits on your FBA account. The main drivers are your sell-through rate, stranded inventory percentage, excess inventory, and in-stock rate.
Stranded inventory is FBA stock that has no active listing — usually because the listing was suppressed, closed, or incorrectly merged. You pay storage fees on stranded units but cannot sell them. Amazon will begin automated removals if it is not resolved within 30 days.
Calculate a reorder point based on your actual daily sell-through rate and supplier lead time. Set a minimum days-of-stock threshold that accounts for transit time to FBA, and use real-time alerts so you know the moment your available count hits zero.
Amazon’s algorithm uses recent sales velocity as a ranking input. A stockout of 3-5 days can cause a meaningful BSR drop that takes weeks to recover. For seasonal or high-traffic ASINs, a weekend stockout during peak periods can knock a listing off page one at the worst possible time.
The fastest IPI fixes are resolving stranded inventory (weekly review in Manage Inventory) and creating removal orders for clearly excess units. Both affect your stranded rate and excess inventory percentage directly. Tightening reorder cycles to reduce stockouts improves your in-stock rate over the following 30 days.
Sellers with IPI scores below 400 face monthly FBA storage capacity limits, expressed in cubic feet. Amazon allocates capacity monthly and notifies you in your Shipping Queue. The most common trigger is carrying excess inventory post-Q4, when storage fees are high and velocity drops. Removal orders or liquidation are the fastest ways to free capacity.
Nisha Shetty · Marketing Manager, SentryKit
Nisha is a marketing manager and former Amazon seller who writes about e-commerce growth, consumer behavior, and digital retail trends.
