FBA Storage and Aged Inventory Calculator
FBA monthly storage fees are charged on the cubic feet your packaged units occupy, multiplied by a rate per cubic foot per month that is far higher during the October to December peak window than for the rest of the year. Units that sit beyond the aged inventory threshold take an aged inventory surcharge in addition to ordinary storage, not instead of it. This calculator converts your unit dimensions into cubic feet, applies your off-peak and peak rates across the months you expect to hold stock, adds the surcharge on the aged portion, and returns total storage exposure and cost per unit stored. Enter your own numbers below; the fields load with a worked example already filled in.
Calculate storage and aged inventory cost
How the storage fee calculation works
Every storage figure Amazon bills you starts from volume. Weight decides the fulfillment fee; volume decides the storage fee, and the two can point in opposite directions for the same product:
- Cubic feet per unit = length × width × height ÷ 1,728. There are 1,728 cubic inches in a cubic foot, so this is simply a unit conversion applied to the packaged dimensions, measured with the polybag, insert, and any retail box in place.
- Total cubic feet = cubic feet per unit × units stored. Amazon works from the average daily volume across the month, so a shipment that arrives mid-month is billed for roughly half of that month.
- Off-peak storage cost = total cubic feet × off-peak rate × months held outside the peak window.
- Peak storage cost = total cubic feet × peak rate × months held inside the October to December window. The rate is the only thing that changes; the volume calculation is identical.
- Aged surcharge = (aged units × cubic feet per unit) × aged rate × months those units remain aged. The surcharge is levied on the cubic feet of the aged portion only, but those same cubic feet are still being billed ordinary storage at the same time.
- Total storage exposure = off-peak cost + peak cost + aged surcharge, and cost per unit stored = that total ÷ units stored. The per-unit figure is what belongs in the storage field of the FBA profit calculator.
Because the fee is volumetric, packaging is the lever with the highest return and the shortest lead time. Reducing a packaged unit by half an inch in each dimension can strip a fifth or more off the cubic feet and therefore the same share off every storage line above, permanently, on every unit you ever ship. No negotiation with a supplier and no repricing move produces that kind of structural saving. It is also the change most sellers never make, because the box was designed once at launch and never revisited.
The seasonal multiple is the second thing to internalize. The peak rate that applies from October through December has historically run at close to three times the off-peak rate, which means a cubic foot held through the fourth quarter costs about as much as three cubic feet held in the spring. That changes the arithmetic on Q4 buying: inventory that arrives in September and sells in November is cheap, and inventory that arrives in September and sells in February is expensive twice, once at the peak rate and again through the months it lingers afterward.
Aged inventory surcharges are the part sellers most often model incorrectly. They do not replace monthly storage, they sit on top of it, and the surcharge bands escalate the longer a unit stays. A slow SKU can therefore be paying ordinary storage, a peak-rate multiple, and an aged surcharge in the same December, which is how a modest quantity of dead stock generates a storage invoice out of all proportion to its unit count. When that happens, the decision is usually between a discount deep enough to clear the units and a removal or disposal order, and the calculation above tells you how much time you have to make it.
The real cost of overstocking is broader still. Add the peak rate and the aged surcharge to the capital sitting idle in the warehouse, the capital that cannot fund the next purchase order or the next product launch, and the storage fee stops being the headline number. That is the frame to use when a supplier offers a discount for a larger order: the discount is certain, and the storage, surcharge, and cash cost of the extra months is the price of taking it. Set your order size from the reorder point calculator and check the fee schedule changes covered in FBA fee changes before assuming last year's rates still hold.
Worked example: 2,000 units of a 9 by 6 by 2.5 inch product
These are the values the calculator loads with, so you can see the method and the widget agree. Figures are illustrative.
- 2,000 units measuring 9 × 6 × 2.5 inches, held 2 months at an off-peak rate of $0.78 and 1 month at a peak rate of $2.40, with 300 units past the aged threshold taking a $1.50 surcharge for 2 months.
- Cubic feet per unit: 9 × 6 × 2.5 = 135 cubic inches, ÷ 1,728 = 0.0781 cu ft.
- Total cubic feet: 0.078125 × 2,000 = 156.25 cu ft.
- Off-peak cost: 156.25 × $0.78 × 2 = $243.75. Peak cost: 156.25 × $2.40 × 1 = $375.00.
- Aged surcharge: 300 units × 0.078125 = 23.4375 cu ft, × $1.50 × 2 = $70.31.
- Total storage exposure: $243.75 + $375.00 + $70.31 = $689.06, or $0.34 per unit stored.
Note what one month at the peak rate does: it costs $375.00 against $243.75 for two full months off-peak. The 300 aged units are 15% of the inventory and generate 10% of the bill on top of the storage they are already accruing.
Frequently asked questions
How does Amazon calculate monthly FBA storage fees?
Storage is billed on the volume your inventory occupies, not its weight. Amazon measures the packaged unit, converts it to cubic feet, multiplies by the average number of units held during the month, and applies a rate per cubic foot per month that varies by size tier and by season. A light but bulky product can therefore cost far more to store than a heavy compact one.
How much higher is the peak season storage rate?
The October to December rate has historically run at roughly three times the off-peak rate for standard-size inventory. That multiple is the reason a Q4 overbuy is expensive twice over: you pay the higher rate on every cubic foot you are holding, and you pay it in the same months your cash is already committed to advertising and restocking.
Does the aged inventory surcharge replace the normal storage fee?
No. It stacks on top. Units that sit past the aged threshold keep accruing ordinary monthly storage at the prevailing seasonal rate and then take the aged surcharge as an additional charge on the same cubic feet. This calculator treats them as separate lines for that reason, which is why the aged units appear in both the storage figure and the surcharge figure.
What is the cheapest way to reduce an FBA storage bill?
Shrink the box before you shrink the order. Because the fee is volumetric, trimming a packaged unit from 9 by 6 by 2.5 inches to 8 by 5 by 2 inches removes about 41% of the cubic feet and therefore about 41% of the storage cost, with no effect on how many units you can sell. Packaging changes also frequently move a product into a cheaper fulfillment size tier.
What does overstocking really cost beyond the storage fee?
The storage line is the visible part. The larger cost is the working capital sitting in a warehouse instead of funding the next purchase order, plus the peak rate applied to units that will not sell until January, plus the aged surcharge if they linger, plus the removal or disposal fee if they never sell. Model the total before deciding that a volume discount from a supplier is worth taking.
Sources
- Amazon Seller Central: monthly inventory storage fee rates, peak window dates, and the aged inventory surcharge schedule (primary source for the rates to enter above).
- Seller Signal analysis. All example rates and figures above are illustrative planning values, not current published rates.