MER stands for Media Efficiency Ratio.
MER is a metric that shows you how much you get back from each dollar (or pound etc) that you spend on ads.
It’s like ROAS, except combining results from all of your ad spend. An eROAS if you will (or a Blended ROAS or ecosystem ROAS).
The equation is:
MER = Total Revenue ÷ Total Ad SpendGlossary Index