ACOS is ad spend divided by ad-attributed sales, so it only measures advertising in isolation. TACOS is ad spend divided by total sales including organic, so it measures what advertising costs your whole business. A falling TACOS with steady revenue means organic rank is carrying more of the load — that is the healthiest signal in an Amazon account.
Why ACOS alone can mislead you
You can improve ACOS simply by switching off every campaign except your branded terms. ACOS drops beautifully and total sales fall off a cliff, because you stopped acquiring new customers.
ACOS answers 'were these ads efficient'. It cannot answer 'is the business growing', which is why it should never be read on its own.
What TACOS tells you
TACOS falling while revenue holds or grows means organic sales are increasing relative to paid — your ranking is improving and you are becoming less dependent on ads.
TACOS rising while revenue is flat means you are buying the same sales at a higher price. That is the early warning of listing decay or a new competitor bidding on your terms.
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