Blended ROAS exists because every advertising platform reports on its own contribution, using its own attribution rules, and none of them know about each other. The predictable consequence is that their claims overlap. Add them up and you get a number larger than the business took.
That is not dishonesty. It is three referees each awarding the same goal.
Where the overlap comes from
- The same sale, twice. A customer sees a Meta ad, then searches your brand and clicks a Google ad. Both claim it.
- View-through conversions. Somebody scrolled past your ad and bought a week later from an email. The platform counts it.
- Demand you already had. Branded search and remarketing capture people who were going to buy anyway. Performance Max is particularly good at absorbing this and reporting it as new business.
- Different windows. One platform counts seven days post-click, another counts a day of views too. They are measuring different things and reporting them in the same column name.
Blended ROAS: the number that cannot be gamed
Marketing efficiency ratio: total revenue divided by total advertising spend. Not revenue a platform claims — revenue the business actually recorded. Not spend on one channel — all of it.
It cannot double-count, it does not care about attribution windows, and it reconciles with your accounts. If you track one acquisition number at business level, this is the one.
Platform ROAS is for choosing between two ad sets. Blended MER is for deciding whether to spend more money. Problems start when the first is used for the second job.
Both numbers have a job
Inside a channel, platform data is the best signal you have. The bias is at least consistent across ad sets, so comparing one creative against another is valid. Use it there, every day.
At business level, use blended. Budget decisions, channel decisions, anything you would put in front of a board.
The test that settles any argument
Raise spend meaningfully on one channel for a fortnight and watch total revenue, not claimed revenue.
- If total revenue rises roughly in proportion, the channel is genuinely incremental.
- If claimed revenue rises and total revenue does not move, you bought attribution, not customers.
The second outcome is much more common than anyone expects, particularly on remarketing and branded search. It is also the single most useful experiment most advertisers have never run.
What a good MER looks like
Entirely dependent on your margin. A brand with seventy percent gross margin survives comfortably at 3x. One with twenty-five percent does not. Work out your break-even MER from contribution margin first, then judge the number you have.
Which means you need to know what an order genuinely contributes after cost of goods, shipping, fees and returns — the contribution margin calculator works that out, and the blended ROAS calculator shows the over-claim in one screen.
The reporting habit
Put blended MER at the top of the weekly report and platform ROAS underneath it. The order matters more than it sounds — whatever is at the top is what people discuss, and the conversation you want is about the business rather than about the dashboard.
The same divergence is why a ROAS figure in a case study needs a follow-up question about which number is being quoted.
Questions people ask
What is blended ROAS?
Total revenue divided by total advertising spend across every channel, taken from your own sales data rather than from any platform. Because it uses one numerator and one denominator, no platform can claim the same sale twice inside it.
What is MER?
Marketing efficiency ratio — the same idea stated as total revenue over total marketing cost. It is the number a finance team recognises, and it moves only when the business actually moves.
Why is platform ROAS always higher?
Because each platform counts every conversion it can connect itself to, and several platforms touch the same customer. The gap widens as you add channels and as you spend more, which is exactly when a misread is most expensive.
Which number should I use for decisions?
Blended for budget decisions and for judging whether the business is better off. Platform ROAS for relative comparison inside one channel — which campaign, which creative — where the attribution rules are at least consistent.