Amazon FBA vs FBM Calculator

How the number is derived

The comparison is contribution per unit under each model. FBA: price − referral − FBA fulfilment fee − storage − cost of goods. FBM: price − referral − your own shipping − your own pick and pack − cost of goods.

The referral fee and the cost of goods are identical in both, so the decision turns entirely on whether Amazon fulfils a unit more cheaply than you can, and on what the Prime badge is worth in conversion.

That second term is the one that usually decides it and the one that cannot be read off a fee schedule.

An example

FBA on the $29.99 product: $4.50 referral, $4.25 fulfilment, $0.09 storage, $8.50 cost, $12.65 contribution.

FBM on the same product: $4.50 referral, $8.50 cost, plus your own shipping. At the parcel economics from the shipping cluster, $9.85 to zone 5 residential, $0.95 packaging, $2.40 labour, that is $13.20 of fulfilment against Amazon's $4.34.

FBM contribution is $3.79 against FBA's $12.65. Amazon fulfils this unit for a third of what it costs you, which is what its network scale buys.

The picture reverses on heavy or bulky items, where FBA's size-tier fees climb steeply and your negotiated freight rates may beat them, and on very slow-moving items where storage and aged surcharges make FBA expensive regardless of the fulfilment fee.

What the number hides

The comparison ignores the Prime badge, which is not a cost line but is frequently the largest single variable. Listings without it convert materially worse on the same product at the same price, and the Buy Box competition is harder to win.

Seller Fulfilled Prime exists but carries strict performance requirements: same-day handling, weekend shipping, and delivery speed standards, that are difficult for a small operation to hold consistently.

FBM also puts customer service and returns on the seller, which is real cost and real time that a per-unit comparison does not capture.

What this changes

Model both properly, including your own fully loaded fulfilment cost rather than just postage. Most FBM comparisons understate the seller's own cost by leaving out labour, packaging and the returns handling.

Then treat the answer as per-product rather than per-account. Most established sellers run both: FBA on the fast-moving small items where Amazon's network wins, FBM on the heavy, bulky, slow or high-value items where it does not.

The hybrid is usually the right answer and it is the one least often chosen, because sellers pick a model at the account level and apply it everywhere.

The cases where FBM clearly wins

Oversize and heavy items, where FBA fees climb into double digits and a negotiated freight or LTL rate is cheaper. Very slow movers, where storage and aged inventory surcharges exceed any fulfilment saving. High-value items where FBA's reimbursement caps sit below the product's actual value.

Custom, made-to-order and personalised products, which cannot sit in a fulfilment centre at all. And products with a genuine unboxing component, where the presentation is part of what the customer bought.

The other case is capital: FBA requires stock to be bought, shipped and sitting in a warehouse before a single sale, while FBM lets a seller hold inventory closer to demand or ship from an existing operation. For a business short of working capital that difference can matter more than the per-unit economics, and it is a legitimate reason to accept a worse fulfilment cost.