Both numbers divide advertising spend by sales. They differ in which sales go in the denominator, and that single difference is why one of them can look healthy while the business underneath is not.
ACOS: spend over attributed sales
ACOS = ad spend ÷ sales Amazon attributed to those ads.
Attribution is the key word. Amazon counts a sale as ad-driven when a shopper clicked (or, in some placements, merely saw) an ad and then bought within an attribution window. Everything else — the customer who searched your brand directly, the repeat buyer, the one who found you through an organic result — is invisible to this ratio.
That makes ACOS the right tool for exactly one job: judging a campaign against another campaign. Which keyword is cheaper to convert? Which ad group is bleeding? ACOS answers that, because both sides of the fraction come from the same advertising system.
TACOS: spend over everything you sold
TACOS = ad spend ÷ total sales.
Same spend, wider denominator. Now the organic sales, the brand searches and the repeat customers are all in there — which is the point. TACOS asks a business question rather than a campaign question: what share of my revenue is going to advertising?
Why the two can disagree
A campaign can hold a perfectly respectable ACOS while TACOS climbs quarter after quarter. That happens when advertising stops being an accelerator and starts being life support: ad-driven sales stay flat and profitable on paper, but they are a growing slice of a shrinking organic base. ACOS cannot see that, because organic sales never enter its arithmetic.
The reverse also happens. A launch campaign with an alarming ACOS can be the right call if it drives the rank that produces organic sales later — and TACOS is the number that eventually shows whether it worked.
How to read them together
Use ACOS to decide where the ad budget goes. Use TACOS to decide whether the ad budget is the right size. A falling TACOS on flat spend means organic sales are growing — usually the healthiest signal a listing can give you.
What neither one includes
Both ratios stop at sales. Neither subtracts referral fees, fulfilment fees, storage, returns or your cost of goods — so neither is a margin, and a "profitable ACOS" can still lose money on every unit. Advertising efficiency and unit economics are two separate questions, and conflating them is one of the more expensive mistakes on this platform.
There is also a timing trap. A sale advertised today can be returned inside the return window, and the refund lands in a later period than the ad spend. Any ad ratio measured over a short, recent window is therefore optimistic by construction.