Break-Even ACOS
The ACOS at which an incremental advertised sale makes exactly zero profit, equal to your gross margin percentage.
Why it matters
You cannot judge whether an ACOS is good without knowing where break-even sits for that specific product. Break-even ACOS equals your gross margin after Amazon referral fees, FBA fees, COGS and inbound freight. A product with a 40% margin breaks even at 40% ACOS; a thin-margin product might break even at 18%.
In practice
This is why blanket ACOS targets across a catalog are a mistake. Two ASINs at the same 30% ACOS can be one comfortably profitable and one quietly losing money on every click, purely because their cost structures differ.
Calculate it per ASIN, not per account, and recalculate whenever Amazon changes fee schedules or your landed cost moves.
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