"My ROAS is 4x, my campaign is doing fine." That sentence can be true and, at the same time, hide a business that's losing money. ROAS, MER and CAC measure different things, at different levels, and confusing them is one of the most common causes of bad budget decisions inside a performance marketing operation.
The three metrics, defined without ambiguity
| Metric | Formula | Level it applies to | What it answers |
|---|---|---|---|
| ROAS (Return on Ad Spend) | Attributed revenue / Ad spend | Specific campaign or channel | Is this campaign generating more revenue than it costs? |
| MER (Marketing Efficiency Ratio) | Total business revenue / Total ad spend | Whole business | Is my marketing investment, as a whole, efficient? |
| CAC | Total spend / New customers | Business or channel | How much does each new customer cost me, compared to what that customer is worth? |
Why a high ROAS can still be unprofitable
ROAS measures revenue, not profit. If a product's gross margin is low, a much higher ROAS is needed for anything to be left over after paying for product cost, shipping and platform fees.
| Illustrative case | Gross margin | Campaign ROAS | Profitable after margin? |
|---|---|---|---|
| Business A | 20% | 4.0x | No, needs a minimum of 5x to break even |
| Business B | 45% | 2.5x | Yes, this business's breakeven is 2.2x |
Illustrative examples for explanatory purposes only; they do not correspond to real accounts. Business A has a numerically higher ROAS, but loses money on every advertised sale; Business B, with a "lower" ROAS, is profitable. The difference is entirely down to margin.
How to calculate your minimum viable ROAS (breakeven)
If a product leaves a net margin of 25% after product cost, shipping and fees (not counting advertising), the breakeven ROAS is 1 ÷ 0.25 = 4x. Any campaign below that number is, in effect, being subsidized by another part of the business or eating into margin. This calculation should be the reference number in every campaign report, not a generic, "aspirational" ROAS.
MER: the metric that avoids the attribution trap
Campaign level ROAS depends on how each platform attributes a sale, and platforms tend to over attribute sales to themselves (the same sale can appear as "generated" in both Meta's report and Google's). MER avoids that problem because it compares the business's actual total revenue against total ad spend, without depending on which platform claims the credit. That's why MER is the right metric for business level budget decisions, while campaign level ROAS remains useful for tactical decisions (which specific campaign to optimize or pause).
As a general market reference, not a fixed target for every business, multiple e commerce analyses place a healthy MER in an approximate range of 3x to 5x, though the right number always depends on each business's margin.
Monthly metrics checklist
- MER for the period, compared against the previous month and against the same month last year if there's seasonality.
- CAC by main channel, compared against the target CAC derived from margin (see How much to invest in digital advertising).
- Breakeven ROAS recalculated if product cost, shipping or fees changed.
- Estimated LTV, if there's enough repurchase or recurrence history to calculate it with confidence.
Frequently Asked Questions
What's the difference between ROAS and MER?
ROAS measures the return of a specific campaign or channel; MER measures the efficiency of the business's entire ad spend against total revenue. MER avoids the cross platform attribution problems.
How do I know if my ROAS is good enough?
By calculating your breakeven ROAS: 1 divided by your net margin available for advertising. Any ROAS below that number isn't profitable, even if it looks high in absolute terms.
What's a good MER?
As a general industry reference, a MER between 3x and 5x is usually considered healthy in e commerce, though the right number depends on each specific business's margin.
Does CAC replace ROAS?
No, they're complementary: ROAS measures revenue efficiency per campaign, CAC measures the cost of acquiring a customer compared against what that customer is worth (LTV). Both are needed to judge profitability.
Conclusion
ROAS, MER and CAC don't compete with each other: they answer different questions at different levels. The mistake isn't using one of these metrics, it's using only one and making budget decisions as if it answered everything.
Not sure if your current ROAS is actually profitable? Book a free audit and we'll calculate your real breakeven ROAS together. Book a call →
Recommended internal links
- What is performance marketing
- Performance marketing for e commerce
- How much to invest in digital advertising
- How to lower your CAC
- Keys to high performance
- Attribution in digital advertising
- How to read your agency's report
- The law of diminishing returns in advertising
- Why your brand only sells with promotions