Short answer
ROAS is often the platform's own attributed revenue for a single channel — and it double-counts across channels. MER is your total revenue against your total marketing spend, with no attribution. For budget decisions across channels, MER beats platform ROAS.
Several channels can claim the same purchase.
All 100 purchases in this example are claimed by at least one channel. Adding channel reports counts shared purchases twice. MER uses store revenue and total marketing spend; it does not determine causal lift.
Why platform ROAS lies about the total
Meta, Google and TikTok each claim credit for the same sales using their own generous conversion windows. Add up their reported revenue and you can hit 130% of what actually came in. Platform ROAS is fine at the ad level, but misleading when you want to know whether the whole machine is making money.
MER cuts through it: total revenue divided by total spend. No attribution, no double-counting. Just what came in and what it cost.
Several channels can claim the same purchase.
All 100 purchases in this example are claimed by at least one channel. Adding channel reports counts shared purchases twice. MER uses store revenue and total marketing spend; it does not determine causal lift.
Use each for its own question
ROAS answers "how does this channel or campaign look through the platform's lens." MER answers "is the total marketing effort making money." For day-to-day optimization, platform numbers are useful; for budget decisions across channels and for scaling, MER is the reference point.
A healthy practice: scale by MER, optimize by POAS/ROAS within channels, and validate with incrementality. That way you're not chasing a flattering platform number that never shows up in the bank.
Frequently asked questions
When should I use MER instead of ROAS?
For budget decisions across channels and for scaling. Platform ROAS double-counts, so it suits optimization within a single channel, but MER is the honest measure of whether the whole effort is making money.
Is a MER lower than platform ROAS a problem?
No, it's expected — MER isn't inflated by attribution. What matters is whether MER holds or rises as you scale. If it falls, you're buying sales you'd have gotten anyway.
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About the author
Nicklas Segatz Mortensen
Growth hacker and fractional CMO with 10+ years' experience and hundreds of millions in managed ad spend behind him. Background from larger Danish and international scale-ups, and from the agency world.
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