How Amazon FBA profit is actually calculated
Every FBA profit calculation is the same subtraction, and most sellers get it wrong by leaving one term out:
Net profit = Selling price − Referral fee − FBA fulfillment fee − Monthly storage − Cost of goods − Ad spend
Amazon shows you the first three. Cost of goods is the one number Amazon cannot know, and it is the one that decides whether a product is worth selling (see how to calculate COGS with FIFO). A calculator that assumes your landed cost is just the invoice price will overstate your margin on every unit.
The four fees that come out of every sale
Referral fee. Amazon's commission on the sale price, charged on every unit regardless of how you fulfill. Most categories sit at 15%, with the full range running roughly 8–20%. It is calculated on the total selling price including shipping — not on your profit — so a price increase raises this fee too.
FBA fulfillment fee. A flat per-unit charge for picking, packing and shipping. It is set by size tier and unit weight, not by price, which is why the same fee can be trivial on a $40 product and fatal on a $12 one.
Monthly storage. Charged on the cubic feet your inventory occupies, and it roughly triples in Q4. Standard-size inventory costs $0.78 per cubic foot from January to September and $2.40 from October to December. Oversize runs $0.56 and $1.40 respectively.
Returns and refund administration. The fee sellers forget. When a unit is returned you lose the sale, and you do not get the full fulfillment fee back. On a category with a 10% return rate, this quietly removes a meaningful slice of margin that never appears in a per-unit calculation.
A worked example
A Large Standard product weighing 12 oz, selling at $24.99 in a 15% referral category, costing $6.20 landed:
Now add advertising. At a 25% ACoS on that $24.99 price, ad spend is $6.25 per unit sold and net profit falls to $4.55 — margin drops from 43.2% to 18.2%. This is why a product that looks healthy in a profit calculator can still lose money in practice, and why it is worth checking your break-even ACoS before scaling ads. Per-campaign ACoS only tells you part of it — TACoS measures how much of the whole business depends on ad spend.
2026 FBA fulfillment fees by size tier
These are the US rates this calculator applies automatically:
Size tier is determined by dimensions and weight, and the boundary matters more than the rate. A product that crosses from Small Standard into Large Standard picks up roughly $0.50 per unit for a fraction of an inch. Before you finalize packaging, check which side of the line you are on — it is one of the cheapest margin improvements available.
What sellers most often get wrong
- Using invoice cost instead of landed cost. Inbound freight, prep, labels and duty belong in cost of goods. Leaving them out typically overstates margin by 3–8 points.
- Ignoring the return rate. A 12% return rate in apparel is not an edge case, it is the base case. Model it or your annual numbers will not reconcile.
- Using a blended average cost across purchase lots. If you bought the same SKU at $4.10, $5.30 and $6.80, an average tells you the product is fine while the most recent batch is losing money. Lot-level FIFO costing is the fix.
- Forgetting Q4 storage. Modeling a Q4 product on the $0.78 off-peak rate understates storage by roughly three times in exactly the months you hold the most stock.
- Confusing margin with ROI. Margin is profit over revenue; ROI is profit over what you spent. A 20% margin product with a fast turn can beat a 40% margin product that sits for six months — ROI and turn rate together tell you which.
Margin or ROI — which should you use?
Use margin when comparing products at similar price points, or when you need to know how much room a price cut leaves. Use ROI when deciding where to put your next purchase order, because it answers the question that actually constrains most sellers: what does this do per dollar of capital? Retail and online arbitrage sellers generally work to an ROI floor — commonly 50–100% — while private label sellers tend to manage to margin.
This calculator estimates one unit at one price. It cannot see what your inventory actually cost across different purchase lots, what Amazon refunded, or which reimbursements you are owed. Those need order-level data — which is what SellerGuards pulls automatically from Amazon's SP-API.