You have three numbers on your screen: Meta says your ROAS is 4.2×. Google says 3.8×. Your spreadsheet says your total revenue divided by total spend is 2.1×. Which one should you use to decide whether to increase your budget?
The answer is the 2.1×. But to understand why, it helps to know exactly what each number is actually measuring, and why they can't simply be averaged or summed.
Platform ROAS: useful inside a platform, misleading across them
Platform ROAS is what each individual ad platform reports: revenue attributed to that platform's ads, divided by what you spent there. The formula is straightforward. The problem is in the numerator.
When a customer clicks a Meta ad on Monday, a Google ad on Wednesday, and buys on Friday, both platforms claim the conversion. Meta attributes it within its 7-day click window. Google attributes it within its 30-day click window. Your payment processor records one sale. The platforms have each told the truth about their own data, but if you add their reported revenues together, you get a number that's larger than what you actually made.
This overlap is why you can never sum platform ROAS figures across channels to get a total picture. Each platform is measuring from its own perspective, with no visibility into what the others are reporting.
Where platform ROAS is useful: optimizing within a single channel. Comparing two creative variants on Meta, deciding which audience to scale, or testing a new campaign structure. Within one platform, where the attribution logic is consistent, ROAS gives you a directionally reliable signal.
Blended ROAS: the honest top-line
Blended ROAS uses a different approach entirely. Instead of asking each platform what it thinks it drove, you look at your actual revenue (from your payment processor) and divide by your total ad spend:
Blended ROAS = Total revenue ÷ Total ad spend
Because the numerator comes from your payment data rather than platform attribution, this number can't be inflated by double-counting. When it goes up, something in your marketing actually improved. When it goes down, something is genuinely wrong. It's the number to anchor your budget decisions to.
MER: blended ROAS for the full marketing investment
Marketing Efficiency Ratio (MER) uses the same formula as blended ROAS, but the denominator expands to include all marketing spend, not just paid ads. That means tools, agency fees, content production, email platform costs, and anything else you spend to acquire or retain customers:
MER = Total revenue ÷ Total marketing spend
Because MER counts more costs, it will always be lower than blended ROAS. A business spending $20,000/month on ads but another $10,000 on agency fees and tools has a very different true efficiency than the ad spend alone suggests.
MER is the right number to use when you want to understand the full cost of driving revenue through marketing, not just paid media.
A worked example
Take a hypothetical business running ads on two platforms in a given month:
- Meta spend: $10,000 (Meta reports $42,000 in attributed revenue, 4.2× ROAS)
- Google spend: $8,000 (Google reports $30,400 in attributed revenue, 3.8× ROAS)
- Actual revenue from payment processor: $37,800
- Additional marketing costs (agency, tools): $4,000
If you summed the platform-reported revenues you'd get $72,400 in attributed revenue, nearly double what actually came in. The real numbers:
- Blended ROAS: $37,800 ÷ $18,000 = 2.1×
- MER: $37,800 ÷ $22,000 = 1.72×
Both platforms looked healthy. The actual return on marketing investment told a different story, one that would lead to a very different budget conversation.
Which to use when
- Platform ROAS: Use for in-channel optimization: comparing creatives, audiences, and campaigns within a single platform. Never use to justify total budget or compare channels.
- Blended ROAS: Use as your primary marketing health metric. Review it monthly and when making budget allocation decisions. If your blended ROAS drops, something real changed. Investigate.
- MER: Use when you want the full picture of marketing efficiency, especially if you have significant non-ad spend (agencies, tools, content). MER makes the true cost of customer acquisition visible.
The practical goal is to watch all three together: platform ROAS for channel-level signals, blended ROAS as the business-level anchor, and MER as the honest total. When the three diverge sharply, that gap tells you something important about where your attribution is leaking.
Where Cavor fits in
Cavor calculates blended ROAS and MER automatically by pulling spend data from your ad platforms and reconciling it against actual payments from your processor, so the number you see reflects what you actually made, not what the platforms claim they drove.