If you’ve ever added up the ROAS each ad platform reports and compared it to your actual business revenue, you’ve probably noticed the numbers don’t line up. That’s not a bug in any one platform — it’s a structural result of how platform-reported ROAS and blended ROAS are calculated differently, even though the formula is identical.

Same formula, different inputs

Both platform ROAS and blended ROAS (also called MER — Marketing Efficiency Ratio) use:

ROAS = Revenue ÷ Ad Spend

The difference is entirely in what goes into “Revenue” and “Ad Spend”:

  • Platform ROAS uses the revenue and spend one ad platform (Google Ads, Meta Ads, etc.) can see and is willing to take credit for, based on its own attribution model and window.
  • Blended ROAS / MER uses total business revenue over total marketing spend across every channel combined — see our Blended ROAS / MER Calculator.

Why platform ROAS tends to run higher

A few structural reasons platform-reported ROAS commonly overstates results relative to blended ROAS:

  • Overlapping attribution. If a customer sees ads on two platforms before converting, both platforms may claim credit for the same sale under their own attribution model. Add up each platform’s claimed revenue and you can exceed total actual revenue.
  • Attribution windows. A platform might count a conversion that happens, say, 20 days after a click, using a window your actual accounting doesn’t match.
  • Blind spots. A platform can only report on what it can track. Organic traffic, direct visits, referrals, and other platforms’ influence aren’t part of its number, but might still be sitting upstream of the conversion.

None of this means platform ROAS is “wrong” — it’s answering a narrower question (how did this platform’s reported activity perform, by its own measurement) than blended ROAS is (how did total marketing spend perform against total business revenue).

When to use each

Platform ROAS is useful for:

  • Comparing campaigns or ad groups within the same platform, where the attribution logic is at least consistent.
  • Day-to-day optimization decisions inside that platform’s tools.

Blended ROAS (MER) is useful for:

  • Understanding whether marketing as a whole is working, independent of which channel gets the credit.
  • Comparing performance over time without attribution-model changes (platform attribution logic changes periodically; total revenue and total spend don’t have that problem).
  • Sanity-checking platform-reported numbers that look unusually strong.

A quick example

Suppose a business spends $20,000 across Google and Meta ads combined in a month, and does $120,000 in total revenue. Blended ROAS = $120,000 ÷ $20,000 = 6.0. If Google Ads reports a 4.0 ROAS on its own claimed spend and revenue, and Meta reports a 3.5 ROAS on its own, that doesn’t mean the business is actually getting more than 6x overall — it means the two platforms’ attribution models overlap in claiming credit for some of the same revenue.

The takeaway

Platform ROAS and blended ROAS aren’t competing answers to the same question — they’re answers to two different questions. As covered in Why ROAS Doesn’t Tell You Whether Your Ads Are Profitable, neither one is a profit figure either — both are revenue ratios. Use platform ROAS for in-platform optimization, and blended ROAS as your check on overall marketing efficiency.