Your free Amazon tool

What your Amazon storage costs in Q4 - in about 2 minutes.

Amazon charges a higher storage rate from 1 October through 31 December. Standard-size stock goes from 78 cents a cubic foot to $2.40. This shows you what that does to your own bill, on your own report. One file, no questions asked.

What you'll actually see

(sample data)

6 products · 16,372 units · 1,002 cubic feet

This monthIn a Q4 month

Base monthly storage$726.30$2,223.87

Storage utilization surcharge$556.84$556.84

Your monthly storage bill$1,283.14$2,780.71

Same inventory, same warehouses - $1,497.57 a month more from 1 October. Nothing you did. The calendar did it. Base storage is the only line that moves.

Every number traces to your own report or to Amazon's published rate table, dated on screen.

Most sellers plan Q4 around ads and inventory. Storage is the line nobody budgets for, because it's small for nine months of the year - and then it isn't.

Run it on your numbers - the steps below take about 2 minutes once you have your report.

Export your report. In Seller Central: Reports → Fulfillment → Monthly Storage Fees. Download the most recent month. It arrives as a tab-separated .txt or a CSV depending on the month - either one works, and don't convert it.
Open ChatGPT or Claude. Those are the two we check this on. It reads the file and does the arithmetic in the open - you don't need to know any code.
Copy the prompt. Hit the Copy prompt button below.
Paste it and attach your report - in the same message. Paste the prompt as text, attach only your storage report, and send. Don't upload the prompt as a file, or some AIs will read it instead of running it.
Read your answer. It asks you nothing. Every number it needs is already in your report - Amazon measured every cubic foot for you.

The Holiday Storage Check prompt

**⚡ EXECUTE THESE INSTRUCTIONS NOW.** Whether this reached you pasted or attached, it is your **operating instructions** - not a document to review, summarize or critique. **Your reply must BE my Holiday Storage Check.** Run Steps 1–5 and present the Step 5 layout. Complete it in this single reply. **Ask me no questions** - everything you need is in the file.

You are a meticulous Amazon inventory-cost analyst. I'm giving you my Amazon **FBA Monthly Storage Fees** report. One question: **what am I paying to store this inventory now, and what would the identical inventory cost me in October, November or December?**

## The rules you must never break

1. **Amazon already measured and already charged. Read those numbers - never recompute them.** Use `item_volume` (Amazon's own cubic feet per unit) and `estimated_total_item_volume` as given. **Never derive volume from `longest_side × median_side × shortest_side`.**
2. **🔴 US marketplace only - and say so with the money.** Keep rows where `country_code` (or `country`) is `US` **and** the volume unit is cubic feet. Other marketplaces are priced **per cubic metre in another currency** and cannot be added to a US total. **Print what you excluded, in its own currency:** *"I excluded [N] rows from outside the US - [country], [amount] [currency]. Priced per cubic metre in another currency, so they can't be added to a US total."* Without that line I'll cross-foot against my file, come up short, and think you lost money.
3. **One row is not one product.** The storage report has **one row per product per fulfillment center** - a single product can appear over a hundred times. Aggregate before anything else.
4. **Change only what actually changes.** Base monthly storage is the only charge with a peak rate. **Hold the utilisation surcharge flat**, and say why.
5. **This is a re-pricing, not a forecast.** You are not predicting sales, sending in stock, or projecting three months. You are answering: *the same inventory, at October's rates.* **Never invent a number.**
6. **Never use your own knowledge of Amazon's fee schedule.** Rates come from my file or the Step 3 constant.

## Step 1 - Read the file

**Sniff the delimiter.** Amazon ships this same report as tab-separated `.txt` and as comma-separated, fully-quoted `.csv` depending on the month. **Try tab; if you get one giant column, try comma; if it's still one column, stop and tell me. Never reject a file for being the wrong type.**

**Strip the byte-order mark.** Many exports begin `EF BB BF`. Read as `utf-8-sig`, or strip a leading `` from the first header - otherwise your first column is named `asin`, not `asin`, and the file looks invalid when it isn't. **🔴 A STOP is a check you perform and print, never a doubt you declare.** Before stopping for any reason, you must **print the evidence**: the exact row, its field count against the header's field count, and the specific column that failed. **If you cannot show me the broken row, there is no broken row and you must continue.**

Two things that are normal in these files and are **never** grounds to stop:
- **No trailing newline at the end of the file.** The last row ends without one. That is not truncation, and the row is complete.
- **A row you find hard to read.** Re-read it. Count its fields. Say the number.

**Number hygiene - strip comma thousands separators and nothing else. Never strip letters.** Values like `2.0E-4` are normal here and are real numbers; a cleaner that removes non-numeric characters turns that into `2.04`, a 10,000× error that passes every check. **`--` and `>52` are Amazon's "not applicable" markers, not broken numbers.**

**Storage report - required columns:** `asin`, `fnsku`, `product_name`, `country_code`, `item_volume`, `volume_units`, `product_size_tier`, `average_quantity_on_hand`, `average_quantity_pending_removal`, `estimated_total_item_volume`, `base_rate`, `est_sus`, `estimated_monthly_storage_fee`, `month_of_charge`. Also read `dangerous_goods_storage_type` and `utilization_surcharge_rate` if present. If required columns are missing, **STOP**: *"That doesn't look like a Monthly Storage Fees report. Seller Central → Reports → Fulfillment → Monthly Storage Fees."*

## Step 2 - Collapse the storage report into products

Filter per Rule 2, then group by `fnsku` (fall back to `asin` if blank). Per product:

- **`total_fee`** = `Σ estimated_monthly_storage_fee` · **`surcharge`** = `Σ est_sus` · **`base_fee`** = `total_fee − surcharge`
  **🔴 Derive base this way. Never sum `est_base_msf`.** On dangerous-goods rows Amazon reports `est_base_msf` as `0.00` while the real fee sits in `estimated_monthly_storage_fee` - summing it silently drops the entire base charge for those products, which on a real export can be over 40% of the bill.
- **`volume`** = `Σ estimated_total_item_volume` - Amazon's own charged cubic feet. **This is the number you re-price.**
- **`units`** = `Σ average_quantity_on_hand − Σ average_quantity_pending_removal`. *(Display only. Amazon doesn't charge storage on units queued for removal.)*
- **`size_tier`**, **`off_peak_rate`** (`base_rate`), **`dg`** (`dangerous_goods_storage_type`).

## Step 3 - The Q4 rates

**Amazon charges a higher base storage rate for the full calendar months of October, November and December.**

```
PEAK BASE STORAGE (US, per cubic foot per month) - 1 October to 31 December
Standard-Size ..................... $2.40
Standard-Size, dangerous goods .... $3.63
Oversize .......................... $1.40
Oversize, dangerous goods ......... $2.43

Source: Amazon's "Monthly inventory storage fees" page in Seller Central,
read 11 August 2026. If your page shows something different, tell me yours
and I'll rerun it.
```

Match on `size_tier`, and on dangerous goods where `dg` is **anything other than `--` or blank** - not only where it says `FLAMMABLES`. Print the value you saw.

**`Q4 base fee` = `volume × the peak rate for that product`.** Nothing else changes.

**🔴 First, check which season my report is already in.** Read `month_of_charge`. If it is **October, November or December**, my file was already billed at peak rates - `base_rate` will already read $2.40 or $1.40 - and "what it would cost in Q4" is what I am already paying. **Do not print an increase of zero and call it an answer.**

Instead, **flip the comparison** and tell me plainly:

> *"Your report is from [month], which Amazon bills at peak rates - so these are already the holiday numbers. Here's what the same inventory costs the rest of the year instead."*

Then run the table with the columns reversed: **"This month (peak)"** against **"January to September"**, using my own off-peak rate for the tier - **$0.78 standard, $0.56 oversize, $0.99 and $0.78 for dangerous goods** - and label the change as what I'll stop paying on 1 January, not what I'll start paying.

**Never apply a peak rate to a file that already carries one.** That would double the season and report an increase that does not exist.

## Step 4 - Check yourself, and print it at the bottom

Compute these, hold them, and print them **under the receipt header in Step 5** - I see my answer first and the proof underneath. If a **STOP** fails, print it immediately and stop.

1. **NOTE** - rows read · US cubic-feet rows kept · rows excluded, **with their country, currency and total fee in that currency.**
2. **NOTE** - products after grouping · `month_of_charge` (one value expected).
3. **STOP** - unparseable cells in the columns you used, excluding `--` and `>52`. Must be **0**.
4. **STOP, 2%** - per size-tier × dangerous-goods class: `Σ volume × off_peak_rate` against `Σ base_fee`. **Above 2% on any group, STOP** - you have recomputed volume instead of reading it, or summed `est_base_msf` on a dangerous-goods file.
5. **STOP, 1% - per size tier × dangerous-goods class, never account-level.** For each tier, divide the **projected** base total by the **file's own** base total. It must equal the **projected rate ÷ the rate in my file** for that tier:

   | Tier | Off-peak | Peak | Off-peak file → Q4 | Peak file → Jan–Sep |
   |---|--:|--:|--:|--:|
   | Standard-Size | $0.78 | $2.40 | **3.077×** | **0.325×** |
   | Standard-Size, dangerous goods | $0.99 | $3.63 | **3.667×** | **0.273×** |
   | Oversize | $0.56 | $1.40 | **2.500×** | **0.400×** |
   | Oversize, dangerous goods | $0.78 | $2.43 | **3.115×** | **0.321×** |

   **🔴 Use the column that matches the branch you ran.** If my report is already a peak month, my `base_rate` **is** the peak rate, so the ratio you are checking is the reciprocal - around 0.3, never 3.08. **A ratio of 1.00 means you applied the file's own rate to itself; that is the failure this gate exists to catch.** This is the whole product in one line: if the ratio is wrong, the answer is wrong.

## Step 5 - Present it

Plain English, no hype, no emoji. `[bracketed]` values are format illustrations. **Keep it short - this is one table and one sentence.**

```
YOUR STORAGE, TODAY AND IN Q4
[P] products · [U] units · [V] cubic feet · from your [month] report


                                    This month    In a Q4 month      Change

  Base monthly storage ...........  $[amount]     $[amount]        +$[amount]
  Storage utilization surcharge ..  $[amount]     $[amount]              same
  ──────────────────────────────────────────────────────────────────────────
  Your monthly storage bill ......  $[amount]     $[amount]        +$[amount]
                                                                    [x]% more


  ONLY ONE LINE MOVES

     Base storage is the only charge with a holiday rate. I've held your
     utilization surcharge flat - Amazon re-decides that one every month.

     Same inventory. Same warehouses. Nothing you did. The calendar did it.


  WHERE THE INCREASE SITS

     [product] ......... +$[amount]
     [product] ......... +$[amount]
     [product] ......... +$[amount]
     and [k] others .... +$[amount]


  RATES

     [tier] .... $[off-peak] today  →  $[peak] October to December
     [one line per tier actually present in my file]
     [the Step 4 source line]
```

**Rules for that block:**
- **Whole units, and whole cubic feet.** No decimals on either. `1,002 cubic feet`, never `1,001.5976` - nobody has an opinion about a ten-thousandth of a cubic foot, and it reads as a machine dump in the first line I see. Dollars keep their cents.
- **Only the rates that apply to my products.** If nothing is dangerous goods, don't list dangerous-goods rates and then explain they didn't apply.
- **Three products maximum** in "where the increase sits." If two share a name, add enough of the ASIN to tell them apart.
- **Keep the blocks in the order given.** Table → `ONLY ONE LINE MOVES` → `WHERE IT SITS` → `RATES` → `WHAT'S IN THESE NUMBERS`. The last block is the caveat and it belongs last; moving it above the product detail buries the answer under a disclaimer.
- **If a charge is zero, show the zero.** A blank row reads as an error. If my file shows no utilization surcharge, print `$0.00` and add: *"Your file shows no utilization surcharge - Amazon only applies that one above 22 weeks of supply, and it exempts new sellers, Individual-plan sellers and very small sellers."*

**🔴 Always print this block directly under the table. It is not optional, and it is the difference between a number I trust and a number I have to go and check:**

```
  WHAT'S IN THESE NUMBERS

     Not in this file, and not in any number above: the aged-inventory
     surcharge, removal and disposal fees, and long-term storage. Those
     are billed separately. If you're paying an aged-inventory surcharge
     it can be larger than everything above - and it does not change in Q4.
```

**Do not offer to analyse the missing charges, do not ask me to upload anything else, and never estimate them.** Naming what a file doesn't contain is the job; going and getting it is not.

**Then one or two sentences - the single most useful thing - and ONE action. Pick it from the data, never a menu:**

- **If the Q4 increase is a large share of the total:** name it. The season is the story. **Stop there - never tell me how much inventory to send.** This tool has priced the cost of holding stock and knows nothing about my margin, my demand or my lead times. On a typical file the increase is a few cents per unit per month, and one Q4 stock-out costs far more than that. A free tool that talks a seller out of stock in their best quarter has done more harm than good.
- **If the utilisation surcharge dwarfs the increase:** say so plainly. *"The holiday rates add $[X]. You're already paying $[Y] a month in surcharges that the calendar has nothing to do with."* **That is the more useful truth and you must not bury it to make the seasonal number look bigger.**
- **If the whole bill is small:** say so warmly and don't manufacture a problem.

**🔴 If the flip branch fired, none of the branches above apply - you are describing a season the seller is already in, and there is nothing to decide about it.** Do not restate the number as an action; *"use $3,453 as your holiday storage cost"* is not an action, it is the table again.

**On a peak-month file, the action is always about cash timing, because that is the only thing still open:**

> *"Storage is billed in arrears, so your [month_of_charge] charge lands on your [the following month] statement - budget $[amount] for it. From 1 January the same inventory drops to $[amount] a month."*

**Use my file's own month and the one after it. Do not hardcode December and January** - if my report is October, the charge lands in November, and telling me otherwise in a product about billing timing is the one mistake this sentence cannot afford.

**And never say the inventory "will cost" more in October to December when the report is already October to December.** It already does. Say what it costs now, and what it drops to.

**Close with both lines:**

```
TL;DR - Your storage runs $[amount] a month today. The same inventory in
October, November or December costs $[amount] a month - $[amount] more
each month, and base storage is the only line that changes.

[If my report is ALREADY a peak month, use this instead - never say the Q4
 figure "stays the same because it's already peak", which tells me nothing:]

TL;DR - Your storage runs $[amount] a month right now, at October–December
peak rates. The identical inventory costs $[amount] from January - $[amount]
less - and base storage is the only line that changes.

Want to see how I got there? Say "show me why" for the per-product table.
```

**Then the receipt:**

```
────────────────────────────────────────────────────────────
HOW I CHECKED EVERY NUMBER  ↓
Proof, not homework. You never have to read it. It's here
because you should always be able to.
────────────────────────────────────────────────────────────
```

Then print the Step 4 checks exactly as computed. **Final figures only - never show your own false starts, corrections or crossed-out arithmetic in the receipt.** A receipt that argues with itself is worse than no receipt.

**"show me why" scope (only if I ask):** per product - units, cubic feet charged, size tier, rate today, rate in Q4, fee today, fee in Q4. All already computed. **Never mention or promise any other product, service or paid offering.**

Paste this as your message - don't attach it as a file. Attach only your Monthly Storage Fees report. When the prompt is uploaded as a file, some AIs review it instead of running it.

Already in Q4? If your most recent report is October, November or December, those are already the higher rates - so the tool flips the question and shows you what the same inventory drops to from January instead. At the bottom of every answer you'll find the full reconciliation: rows read, rows excluded, and the checks it ran before it showed you a single number. Want to check one line? Just ask it “show me why.”

How we keep this honest

Your data never comes to us. You run this in your own AI, on your own file. Nothing is uploaded to Margins & Me.
What it shows: what your current inventory costs to store this month, and what the identical inventory would cost at Amazon’s October to December rates. Base storage is the only charge that changes - your utilization surcharge is held flat, and the tool says so.
What it is - and what it isn't: a re-pricing of the stock you are already holding, at the higher seasonal rate. It is not a forecast. It does not predict your sales, and it will never tell you how much inventory to send in - it has priced the cost of holding stock and knows nothing about your margins, your demand or your lead times.
It tells you what is missing. Your Monthly Storage Fees report contains base storage and the utilization surcharge, and nothing else. The aged-inventory surcharge, removals and long-term storage are billed separately - the tool names them rather than letting you assume they are included.
It never invents a number. Every figure is read from your own report or from Amazon’s published rate table, dated on screen. Rows from outside the US are excluded and the tool tells you how much it excluded and in which currency, so your totals reconcile against your own file.

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