What is MER?
The Marketing Efficiency Ratio, also called blended ROAS, divides total revenue by total marketing spend over a period. Every euro of revenue, every euro of spend, no attribution model in between. A MER of 5 means the business made five euros for every euro invested across all channels combined.
Why it matters
Platform ROAS is self reported, and every platform claims the same sale. Add up Meta, Google and TikTok and you get more revenue than you actually have. MER cannot be claimed by anyone. If Meta's reported ROAS doubles and MER stays flat, something was reclassified, not earned. It is the number a CFO will trust before any dashboard.
How to use it
- Fix the period: monthly at minimum, and always the same definition of revenue and spend.
- Split new from returning: a new customer MER tells you what acquisition really costs; the blended figure hides it.
- Watch the trend, not the level: the direction after a budget change says more than the absolute value.
- Pair it with a test: MER moves, a geo test or MMM says why.
The honest view
MER is a thermometer, not a diagnosis. It tells you the whole is healthier or sicker, never which channel did it. Used alone it invites the wrong cut. Used with incrementality testing it becomes the sanity check every attribution report should be measured against.
.svg)
