What is ROAS vs MER?

ROAS = ad revenue / ad spend (single channel). MER = total revenue / total marketing spend (all channels blended). MER is the metric that actually matters; ROAS is the metric Meta hands you.

ROAS = single-channel measurement

Return on Ad Spend = revenue attributed to a channel / spend on that channel. Meta tells you your Meta ROAS. Google tells you your Google ROAS. Each number is platform-attributed, which means it's optimistic and overlapping with other channels.

MER = blended measurement

Marketing Efficiency Ratio = total revenue (all sources) / total marketing spend (all channels). It's the only ad spend metric that can't be gamed by attribution overlap. If MER is 3.0, every $1 of marketing spend produces $3 of revenue regardless of which platform claims credit.

Which to use

Use ROAS for in-platform optimization (Meta's CBO, Google's smart bidding). Use MER for budget decisions, board reporting, and judging whether your overall marketing is working. If platform ROAS goes up but MER goes down, you're seeing attribution shifting — not real lift.

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