Amazon FBA Inventory Management: How Your Monthly Capacity Limit Actually Gets Set

Since March 1, 2023, Amazon has calculated a single monthly capacity limit for your account, and everything you ship in or leave sitting on a shelf has to fit inside that one number. Most FBA inventory advice still reads like a list of habits for raising a score, which leaves you working on a metric without knowing what it buys you. Your Inventory Performance Index score does feed the capacity calculation, but it is one of four inputs, and the other three rarely get named. If you already run a Vendor Central purchase-order cycle, treat what follows as the parallel system on the FBA side: what sets your number, what IPI measures, what sitting stock costs you, and the weekly checks that keep next month's ceiling from landing as a surprise.

The System You Are Operating Inside

FBA capacity management folded two older controls, the weekly restock limit and a separate storage limit, into one consolidated monthly capacity limit for the account. That history matters when you go looking for help, because guidance written before the change treats restock and storage as two separate problems, and plenty of what still surfaces on this topic describes a system that no longer runs.

Your limit is measured in cubic feet, with unit estimates supplied alongside for reference, and it covers both the inventory on hand at fulfillment centers and the inbound shipments that haven't arrived yet. A truck in transit spends capacity the same way a pallet already on a shelf does, so a large replenishment commits against the ceiling the moment you create the shipment rather than the day it is received.

Amazon posts next month's limit in the Capacity Monitor in Seller Central during the third full week of the month and sends an email notification at the same time. Put that week on your calendar, because next month's ceiling gets set before you sit down to build next month's plan. The Capacity Monitor also carries a three-month forward view, which is the part you want when you're sizing a seasonal buy: the current number tells you what you can ship now, and the forecast tells you whether a Q4 plan has anywhere to land. If a plan needs more room than you've been allotted, you can request additional capacity for a reservation fee, and performance credits are designed to offset that fee, up to 100% of it, as the products sell through.

The Four Inputs Behind Your Number

Amazon sets the limit from four things: your IPI score, your sales forecasts, your shipment lead times, and the capacity available across its fulfillment centers.

IPI feeds a blended calculation now, so a weak score pulls your ceiling down without acting as a gate of its own. That distinction matters when you're reading older material: if a guide names a specific IPI threshold as the trigger for a restock restriction, check its date, because the standalone restock limit it describes is the control that capacity management replaced.

The sales forecast is the input easiest to leave stale. Most operators keep one for internal planning and update it when there's time, but Amazon reads it when sizing your limit, which turns truing it up into an operational task with a deadline attached. Shipment lead times work the same way from the other side, rewarding shipments that show up when the plan said they would. Fulfillment-center availability is the one input you can't touch at all, and that is the argument for leaving yourself slack instead of building right up to the last cubic foot.

What IPI Actually Measures

IPI runs on a scale of 0 to 1,000, higher being better, and Amazon makes it available to sellers on a Professional plan who have an active account, recent selling activity, and inventory sitting at its fulfillment centers.

Two of its definitions are worth getting exactly right, because they are the ones operators tend to quote from memory:

• Excess inventory is stock representing more than 90 days of supply measured against customer demand.
• Sell-through rate is units sold and delivered over the past 90 days divided by the average number of sellable units you hold at fulfillment centers.

Both run on a trailing 90-day window, so a month of clean selling won't clear a bad quarter out of the calculation. And read that sell-through denominator for exactly what it says: an average of the sellable units you are holding, so it moves with your stock position as well as with your sales.

Amazon's own FBA Inventory tool in Seller Central is organized around the three moves that follow from all this: planning for demand, reducing excess and aged inventory, and fixing stranded inventory. Work from that screen, since it reads the same records the score does. For tactics on raising the score itself, we've written those up separately in enhancing your Amazon Inventory Performance Index score.

What It Costs to Sit Still

Monthly storage cost is charged on the cubic feet your inventory occupies, and the rate varies seasonally, running higher through the holiday months when space is scarcest. Aged inventory carries a separate charge category on top of that, so slow stock gets billed twice, once for the space and again for the time.

Rates change, and the only numbers that matter are the ones on your own account, so pull the Fee Preview report in Seller Central to see what you're paying today and run any reorder quantity through the Revenue Calculator before you commit to it. While the report is open, find which ASINs carry the largest share of your cubic footage, then check whether their sell-through rate justifies the space they're holding.

Reconcile the Record Before You Plan Against It

Every number above is computed from Amazon's record of what is sellable in its fulfillment centers, right down to the denominator of your sell-through rate. That is reason enough to check the record against your own count before you use it to size a reorder, and it is why fixing stranded inventory sits inside the FBA Inventory tool as one of the three jobs it is built for. If you have open returns or damage claims, our guide for FBA sellers on returns, refunds, and damaged or lost products covers how to work them.

The Weekly Rhythm

Four checks, on a fixed day each week:

1. Capacity Monitor. Read your current limit, how much of it you're consuming, and the forward view. During the third full week, read the new number the day it posts.
2. Sell-through and excess. Compare against last week instead of last month, so a slide shows up while you can still do something about it.
3. Forecast truing. Update the sales forecast before you build the next inbound shipment, since it feeds both your own plan and Amazon's capacity calculation.
4. Stranded and unreconciled units. Check what Amazon records as sellable against what you believe you are holding, and clear anything stranded before next month's plan gets built on it.

If you're running this alongside a Vendor Central account, keep the two ledgers separate. The Capacity Monitor and the IPI dashboard are Seller Central screens governing what you sell through FBA, while the purchase-order side works on different mechanics that we cover in Amazon inventory management on Vendor Central. And if you're still weighing the move, our guide to switching from FBS to FBA covers the setup work that comes first.

Where to Start

Open the Capacity Monitor and write down three numbers: your current limit, how much of it you're using, and the date the next one posts. Then pick the single input you can move before that date, which for most accounts is either excess inventory sitting past 90 days of supply or a forecast nobody has touched since the last planning cycle. Move that one, and when the new limit lands, see whether the number responded.