The maths
ACOS = ad spend ÷ revenue × 100
Margin needed to break even = 1 ÷ ROAS
ROAS and ACOS are the same fact stated two ways: one is a multiple, the other a percentage, and they are reciprocals of each other. A 4× ROAS is a 25% ACOS. Teams sometimes report both as though they were independent measures of performance. They are not.
That reciprocal relationship carries a second, more useful consequence. Because ACOS is 1 ÷ ROAS, your ACOS is the contribution margin you have to beat. At a 3× ROAS, advertising consumed a third of the revenue it produced, so you must keep more than 33% of every order after product, fulfilment and payment costs simply to have made money. Most people quoting a ROAS target have never checked that figure against their own P&L.
Reading it properly
A ROAS figure on its own is not a verdict. It is one half of a comparison, and the missing half is your contribution margin. I have audited accounts running at 8× that were unprofitable because of returns, and accounts at 1.9× that were comfortably in profit on a high-margin digital product.
Treat the ROAS your ad platform reports as a claim rather than a measurement. Platforms attribute revenue generously and count it at the moment of purchase, before refunds. When several platforms each claim the same order, the sum of their reported revenue can exceed what your commerce platform actually recorded.
Common mistakes
- Comparing ROAS across products with different margins. A blended target quietly overspends on your thin-margin lines and starves the profitable ones.
- Reading platform ROAS as company ROAS. Add the revenue claimed by every channel together and compare it against your commerce platform for the same window. The gap is usually instructive.
- Ignoring returns. Revenue booked at checkout is not revenue kept. In categories with meaningful return rates, reported and real ROAS drift apart week after week.
- Chasing a higher ROAS by cutting spend. ROAS almost always rises as budget falls, because you keep only the cheapest, most obvious demand. Higher ROAS with lower total profit is a very common and very expensive win.
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Questions people actually ask
What is a good ROAS?
There is no universal answer, and anyone offering one without asking about your margins is guessing. Your break-even sits at 1 divided by your contribution margin. Above it you make money, below it you lose it. That is the only benchmark that means anything for your business.
What is the difference between ROAS and ROI?
ROAS compares revenue against advertising cost alone. ROI compares profit against total investment, including the cost of goods, salaries and everything else. ROAS is a media efficiency measure; ROI is a business measure. They answer different questions and should not be used interchangeably.
Is ACOS just ROAS upside down?
Yes, exactly. ACOS is ad spend as a percentage of revenue, so a 4× ROAS is a 25% ACOS. Marketplaces such as Amazon tend to prefer ACOS; Google and Meta tend to prefer ROAS. Nothing changes but the framing.
Why does my ROAS fall when I increase budget?
Because the cheapest demand gets bought first. As you raise budget you reach people with less existing intent, so the cost of each additional conversion rises. This is normal and expected. The right question is not whether ROAS fell, but whether the additional profit was still worth having.
Should I optimise for ROAS or for profit?
Profit, always — but ROAS is a usable proxy once you have set the target from your actual contribution margin rather than inherited it. Work out the floor first, then let the bidding strategy chase a number that means something.
Go deeper: How I run performance marketing · Measurement & analytics · Field notes