← Amazon FBA vs FBM calculator
Sellers search for “break-even ACOS” because Sponsored Products is managed as a percent of the sale price, not as a dollar on a spreadsheet. A 30% ACOS target is meaningless if referral, fulfillment, inbound, and the buy cost have already consumed the listing.
Break-even ACOS is the advertising spend that drives net profit to zero after every other cost on the selected fulfillment path. Target-margin ACOS is the lower spend that still leaves the net margin you typed. Both numbers change when you switch FBA to FBM, because storage and inbound are not the same as the label you buy yourself.
Generic ACOS pages often divide leftover margin by price and stop. They miss that FBA and FBM leave different leftover dollars on the same SKU, so they publish one campaign cap for two different cost stacks. Open the Amazon FBA vs FBM calculator, enter the rates from your current fee preview, and read break-even ACOS on each path next to the max buy cost.
The calculator is arithmetic, not advertising advice. It does not set a bid, and it does not claim a campaign will stay profitable after you change the spend.
Break-even ACOS is the advertising spend, as a percent of the sale price, that drives net profit to zero after referral, fulfillment, storage, inbound, and cost of goods on the selected FBA or FBM path.
The leftover dollars change when storage and inbound replace the label you buy yourself. One campaign cap for both paths is two different cost stacks pretending to be one number.