Two metrics, two questions
ROAS answers: how did this channel perform, according to that channel? Revenue attributed by Meta, divided by spend on Meta. MER answers: how did marketing perform, according to the bank? Total company revenue divided by total marketing spend, with no platform in the middle.
MER is also called blended ROAS. Same ratio, different room. Marketers say blended ROAS; finance says MER.
Why the platform numbers do not add up
Run this test once and you will never present platform ROAS to a board again. Add up the revenue Meta, Google and TikTok each attributed last month. Compare it to what Shopify or your accounts say the company actually banked. The platforms' total is usually higher, sometimes by 30% to 60%.
It happens because every platform claims the orders it touched. A customer who clicked a Meta ad, searched your brand on Google and then bought is one order in Shopify and two conversions in the ad accounts. View-through attribution and long click windows widen the gap. Nobody is lying. Each platform is simply reporting on itself, and none of them can see the others.
MER cannot double-count. It has one revenue number and one spend number, and neither comes from an ad platform. The MER calculator shows the over-attribution as a pound figure, which is the most useful slide you will ever put in front of a finance director.
When to use which
| Decision | Use | Why |
|---|---|---|
| Which ad to keep running | ROAS (or POAS) | Comparing two things inside one channel, the platform's own view is consistent |
| How much to spend in total | MER against break-even | Only the blended number reflects what the business banked |
| Whether to add a channel | MER before and after | The new channel's own ROAS will look wonderful and mean little |
| Board and investor reporting | MER, margin-adjusted | It reconciles to the P&L. Platform ROAS never will |
Margin-adjust it or it flatters you too
MER fixes double-counting. It does not fix margin. An MER of 4.0 on a 20% contribution margin is a loss; the same MER on 40% is healthy. So the board number should be MER alongside its break-even, which is 1 divided by contribution margin, or simply MER multiplied by margin, which needs to sit above 1.0.
aMER: the harder version
MER counts every order, including customers who would have bought again without seeing an ad. aMER, acquisition MER, uses new-customer revenue divided by acquisition spend. It is a tougher test and it is the one that tells you whether paid media is actually winning new business or just taking credit for retention. Track both. When MER holds steady while aMER falls, the brand is coasting on its existing customers and the acquisition engine is quietly failing.
The board slide
Five numbers, in this order, each month with the previous month beside it:
- Net revenue, from the accounts.
- Total marketing spend, fully loaded: media, agency, creative, tooling.
- MER, and next to it break-even MER for the month's blended margin.
- aMER, with new-customer share of revenue.
- Platform over-attribution: what the platforms claimed minus what was banked. Watch this line grow as spend grows.
Leave ROAS off the slide. It lives in the marketing team's weekly review, where it is useful, and it dies in the boardroom, where it is not.
Frequently asked
Is MER better than ROAS?
For deciding total spend and for reporting to finance, yes. For deciding between two ads inside one channel, ROAS or POAS is the right tool. They answer different questions and a healthy team uses both.
What is a good MER?
It depends on margin. Divide 1 by contribution margin to get break-even MER; a 30% margin needs 3.3, a 50% margin needs 2.0. A good MER sits comfortably above that floor. Comparing MER to another company is meaningless without their margin.
Why is my MER falling while ROAS is rising?
Usually over-attribution. As spend rises, the platforms claim more of the orders that were going to happen anyway, so platform ROAS holds up while the blended number sags. It can also mean retention is weakening while acquisition looks fine.
Can MER be gamed?
Cutting spend raises MER in the short term because revenue lags. That is why MER is a monthly decision metric and a weekly directional one, never a daily target.
Is MER the same as blended ROAS?
Yes. Both are total revenue divided by total advertising spend. MER is the term finance teams use; blended ROAS is the marketing term for the same ratio.