ROAS vs MER: Which Number Should You Actually Trust?

The short answer For day-to-day decisions inside Meta, use in-platform ROAS. For scaling decisions, use MER (Marketing Efficiency Ratio = total revenue / total ad spend).

Why platform ROAS lies (a little) Meta's pixel sees clicks and view-through within its attribution window. It cannot see your email, SMS, organic, or word-of-mouth. So as you scale paid, platform ROAS often holds while blended quietly drops.

Why MER alone isn't enough MER doesn't tell you which channel is working. A 3.0 MER could mean your paid is crushing it — or that your organic is carrying a wasteful paid program.

The hierarchy that works 1. **Contribution margin** — the final boss. Are you actually making money? 2. **MER / blended ROAS** — directional truth for the whole business 3. **New-customer ROAS** — is paid pulling its weight? 4. **Platform ROAS** — useful for in-account optimization only

Practical move Set a MER floor (e.g. 2.2) and a new-customer ROAS floor (e.g. 1.5). Scale aggressively while both hold. Pull back the moment either breaks.