ROAS is the revenue an ad platform credits to its own ads, divided by what you spent there. MER (marketing efficiency ratio) is your total revenue divided by your total marketing spend across every channel. Use ROAS to compare campaigns, and use MER to decide whether the business is profitable and how much budget it can carry.
ROAS and MER in one minute
ROAS is the revenue a platform credits to its ads divided by what you spent on them. It is useful for comparing campaigns, but every platform grades its own homework, so one customer can be claimed by two platforms.
MER, also called blended ROAS, is total revenue divided by total marketing spend across every channel. It ignores attribution, so it cannot be double counted. It tells you whether marketing as a whole pays, but not which channel earned the result.
A worked example
The numbers are illustrative. A store sells $200,000 in a month and spends $50,000 across three platforms.
| Channel | Spend | Platform ROAS | Revenue claimed |
|---|---|---|---|
| Channel A | $30,000 | 4.4 | $132,000 |
| Channel B | $15,000 | 5.2 | $78,000 |
| Channel C | $5,000 | 4.0 | $20,000 |
| Total | $50,000 | 4.6 blended | $230,000 |
The platforms claim $230,000, but the store only sold $200,000, so some sales were counted more than once. The MER is $200,000 ÷ $50,000 = 4.0, not the 4.6 the platforms imply.
Which metric should you scale on?
Use MER to decide how much the business can afford to spend, and ROAS to decide how to split that spend. If platform ROAS rises while MER stays flat, the platforms are taking credit for sales that were coming anyway. An incrementality test settles it.
To find the MER you need, see our guide to break-even ROAS.
Frequently asked questions
What is a good MER?
There is no universal number. It has to sit comfortably above your break-even MER, which is 1 divided by your contribution margin: 2.0 at a 50% margin, 4.0 at 25%.
Is MER the same as blended ROAS?
Usually. Only the definition of spend can differ, so pick one and keep it fixed.