ACOS is not a verdict.

"Is a 30% ACOS good?" is the most commonly asked question in Amazon advertising and it has no answer. Not a difficult answer, not a nuanced one — no answer, because the question is missing the only input that determines it.

A 30% ACOS on a product with a 39% contribution margin is comfortable. The same 30% on a product with a 25% margin loses money on every order it produces. Same number, opposite conclusion, and nothing in any advertising report tells you which one you are looking at.

The number you are missing

Break-even ACOS is the advertising cost of sale at which one more order contributes exactly nothing. Spend less than it and the sale earns; spend more and it costs you.

Here is the part almost nobody is told: break-even ACOS is the same number as your contribution margin. Not related to it, not derived from it — the same number, answering a different question.

Once you have that figure, the abstract question becomes a concrete one. An ACOS of 28.75% against a 39.11% break-even keeps 10.36 percentage points — about ₹135 a unit. An ACOS of 45% is losing money on every advertised sale, no matter how respectable 45% sounds next to an industry average.

ROAS is the same fact, inverted

Agencies quote ROAS, Amazon reports ACOS, and sellers get asked for both. They are reciprocals: ROAS equals 100 divided by ACOS. A 25% ACOS is a 4× ROAS. Neither carries information the other lacks.

The thing worth watching is that ROAS invites a specific mistake that ACOS does not. ROAS counts revenue returned per rupee spent — revenue, not profit. A 3× ROAS on a product with a 25% margin is a loss: ₹100 of spend produced ₹300 of sales carrying ₹75 of contribution.

Break-even ROAS is 100 divided by your break-even ACOS. On the ₹1,299 unit above, that is 2.56×. The widely repeated "3× is the standard" comes from nowhere in particular, and for a 20% margin product it would be a loss on every sale.

What ACOS structurally cannot tell you

Even with break-even in hand, ACOS has a blind spot it can never cover. It only sees sales the ads were credited with. It cannot see the organic sales advertising caused, and it cannot see the organic sales you would have made anyway.

That matters because the two accounts look identical in ACOS terms: one where advertising is building rank that then sells on its own, and one where advertising has quietly become the only thing selling.

TACoS separates them, by dividing the same spend by total revenue rather than attributed revenue.

One month of TACoS says almost nothing. The signal is entirely in the direction it moves while revenue moves:

• Falling, revenue rising — advertising is compounding. Rank bought earlier is now selling without paying.
• Flat, revenue rising — scaling in proportion. Normal for an established catalogue.
• Rising, revenue rising — buying growth. Fine when deliberate, expensive when it has been true for six months.
• Rising, revenue flat — paying more for the same sales. Something changed: a competitor, a Buy Box loss, or bids drifting up without a decision.
• Above your total net margin — advertising is consuming the business, and this can be true while every campaign shows an acceptable ACOS.

The one that quietly contradicts the others

There is a case worth naming because it looks like good news. ACOS falling while TACoS rises means advertising got more efficient and organic sales shrank faster than spend did.

In other words, the ads are increasingly the only thing selling. A listing lost rank, or reviews, or the Buy Box, and the advertising is holding revenue up. Every campaign report will look fine. Only the two metrics read together show it.

Where to start

Work out break-even ACOS once per product family. It changes when fees, prices, freight or fulfilment method change — not weekly, but not never either.

Then judge campaigns against their product's own figure rather than an account-wide target. A single target across a catalogue overspends on the thin products and starves the strong ones, which is a slower and more expensive mistake than any individual bid.