ACOS Calculator

The underlying calculation

Advertising cost of sale is spend divided by attributed revenue, expressed as a percentage: ACOS = spend ÷ revenue × 100. It is the reciprocal of ROAS.

A 40% ACOS is a 2.5 ROAS; a 55% ACOS is a 1.82 ROAS. The conversion is simply 100 ÷ ACOS = ROAS.

Amazon uses ACOS while most other platforms use ROAS, which is why advertisers running both convert constantly between the two.

Putting numbers to it

$4,000 of spend on $10,000 of attributed revenue is a 40% ACOS.

At a 55% contribution margin, that revenue carries $5,500 of contribution against $4,000 of spend, $1,500 of profit.

Break-even ACOS is the contribution margin itself: 55%. Anything below contributes and anything above loses.

That identity, break-even ACOS equals contribution margin; is the most useful fact in the whole metric, and it makes the profitable threshold immediately readable without any further arithmetic.

Where it is unreliable

Like ROAS it measures attributed rather than incremental revenue, and the same over-claiming applies. Branded campaigns report low ACOS and add little; unbranded campaigns report high ACOS and do the acquisition.

It also treats every sale as standalone, ignoring repeat purchases and the organic rank that advertising builds, both of which are real returns falling outside the attribution window.

How to act on this

Set the target as a fraction of break-even ACOS rather than as an absolute number, and segment branded from unbranded campaigns so each is judged against a different threshold.

Then track total advertising cost of sale alongside it, spend against total revenue including organic, to see whether the advertising is building anything or simply renting sales.

Why ACOS feels more intuitive than ROAS

ACOS is a percentage of revenue, which lines up directly with every other cost percentage a business tracks: cost of goods, fulfilment, overheads. It slots into a profit and loss statement without conversion.

ROAS is a multiple, which is harder to reconcile against a margin without doing the reciprocal in your head. That is why Amazon sellers, who think in unit economics, took to ACOS, and why brand advertisers, who think in returns on investment, took to ROAS.

Neither is better and both describe the same fact. What matters is knowing your break-even in whichever unit you work in, and being able to convert quickly enough that a number quoted in the other form is not meaningless.

Comparing ACOS against the product's gross margin identifies immediately which products can afford to be advertised at all. A product with a 30% margin cannot sustain a 40% ACOS at any volume.

Running that comparison across a catalogue usually finds a group of products being advertised into a structural loss, and stopping those is a faster profit improvement than optimising the ones that work.

Doing that review quarterly keeps the catalogue honest as fees, costs and prices all drift.

Comparing ACOS across products with different referral fee categories also needs care, since the contribution available differs before any advertising decision is made.