Why platform ROAS lies — and the number to track instead

Here’s an experiment. Take one sale. Ask Meta whether its ad caused it. Then ask Google the same thing, about the same sale.

Both will claim it.

That’s not a glitch. It’s how platform-reported ROAS works — and it’s why the number on your ads dashboard is one of the least trustworthy figures in your business.

Every platform marks its own homework

Each ad platform measures conversions through its own attribution window, with its own view of the customer journey, optimised to make its own spend look essential. None of them is lying, exactly. They’re each telling a true story from a flattering angle.

Add the angles together and you get a number that’s bigger than reality. Sum the ROAS your platforms report and it will routinely “explain” more revenue than your business actually made.

So if you’re scaling spend on platform ROAS, you’re scaling on a number that’s structurally optimistic. It feels like growth right up until the bank balance disagrees.

The number that doesn’t flatter

There’s a figure no platform can inflate, because it comes from your own books: contribution margin.

It’s what’s left from a sale after the costs that sale genuinely caused — product, fulfilment, payment fees, platform fees. Not overhead. The direct cost of that order existing.

From contribution margin you can derive the one number every founder should know and most have never calculated: breakeven ROAS. The exact return at which a campaign stops making money and starts losing it. (Our free Easier Profit Calculator exists to hand you that number in about twenty minutes.)

Once you have a breakeven ROAS built from real costs, every campaign sorts itself instantly into “this pays” or “this doesn’t.” No platform gets a vote.

Three layers, not one

Contribution margin tells you if you made money. It doesn’t tell you whether the ad caused the sale. For that you need incrementality, and incrementality needs more than a dashboard:

A daily operating layer — contribution margin, watched live, to run the business day to day.

A quarterly truth layer — geo-lift or holdout tests that isolate what your ads genuinely caused, versus sales you’d have made anyway.

A strategic layer — marketing-mix modelling, to allocate budget across channels over the long run.

Most brands have only the first layer, and they’ve usually outsourced even that to the platforms. That’s the gap. Platform ROAS isn’t a measurement system. It’s a sales pitch with a number attached.

Track contribution margin. Verify with incrementality. And stop letting the people you buy ads from also be the people who grade them.