Google Ads reports ROAS 400%. Meta reports 350%. On each dashboard, both look great. But add the two numbers together to judge "overall ad efficiency" and you're looking at more revenue than actually exists — because each platform counts the same purchase as its own win. That gap quietly inflates how well you think your advertising is doing.
TL;DR
- Platform ROAS gets inflated per channel — each platform counts the same purchase, so summing ROAS exceeds your actual ad-driven revenue
- MER = total revenue ÷ total ad spend — one number for whole-business efficiency, immune to cross-platform double-counting
- ROAS for per-channel tuning, MER for budget decisions — and watch MER when you scale spend
- MER alone still can't tell new from returning — high apparent ROAS often skews to returning buyers who'd have come back anyway
Why summing platform ROAS overstates results
ROAS is a platform's ad-driven revenue ÷ its ad spend. The catch: one customer often touches several ads before buying. See a Meta ad, search on Google, then buy — and both Meta and Google count "that purchase was mine." That's attribution double-counting.
For a store spending ¥1M total (Google ¥500K, Meta ¥500K): Google claims ¥2.0M, Meta claims ¥1.75M — ¥3.75M of claims against ¥3.0M of actual ad-driven revenue. The ¥750K gap is the same purchases counted twice. Summing per-platform ROAS overstates your result by 25%.
What MER is
MER (Marketing Efficiency Ratio) is whole-business total revenue ÷ total ad spend. For that same store, ¥5M total revenue against ¥1M ad spend is MER 500%. Because MER never assigns revenue to a platform, there's no double-counting.
There's a second benefit: non-ad revenue sits in the numerator too. Double your ad spend and if MER drops, that added spend is lowering overall efficiency — a move per-platform ROAS tends to hide, because each platform keeps reporting healthy attributed numbers.
When to use which
Use ROAS for per-channel campaign tuning and creative comparison — it's the daily, actionable number. Use MER to set how much total budget goes where, and to check whether total revenue actually grew when you raised spend.
Refine inside channels with ROAS; decide total spend with MER. Chase per-platform ROAS alone and you miss the state where overall efficiency is falling while every platform still looks fine.
What MER still hides: new vs returning
MER blends everything into one number, so it erases whether revenue came from new or returning customers. Ad platforms push delivery toward whoever buys cheapest — usually returning customers who'd have come back anyway. So channels skewed to returning buyers show high apparent ROAS, while channels reaching genuinely new customers look weak on the spot.
Cut a new-customer channel because its ROAS is low, and you close off the entry point for future repeat buyers. A search channel that harvests existing demand and a social channel that creates new demand shouldn't be judged on the same revenue yardstick. Confirm overall efficiency with MER, but evaluate new-customer contribution separately — by channel, and by new vs returning.
Bottom line
Per-platform ROAS double-counts, so summing it overstates your ad results. MER measures whole-business efficiency in one number and sidesteps that. Refine channels with ROAS, set total spend with MER — and split new from returning so you protect the right channels when budgets tighten.
How does your team handle the platform-ROAS-vs-blended question — do you track MER, or still report channel ROAS separately? Curious how others draw the line.
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