What is MER?

MER (sometimes called blended MER or aMER for the variant that uses only new-customer revenue) is a top-down number you can read straight off your Shopify and ad-account totals: total revenue / total marketing spend. Its strength is that it is un-gameable by attribution. When Meta, TikTok, and Google all claim the same purchase, platform-reported ROAS triple-counts that sale, but MER divides real total revenue by real total spend, so it cannot lie about whether the business as a whole is growing efficiently. That makes MER the metric most operators trust for board-level and budget-level decisions after iOS tracking loss made per-platform ROAS unreliable. Its weakness is the mirror image of its strength: because it blends everything, MER cannot tell you which channel, campaign, or creative is working. It will not catch a single dying ad or a wasteful campaign, and it lags (organic and email revenue that MER credits to this month was often seeded by paid spend in prior months). Use MER to govern total spend and protect margin, and use channel ROAS, CPA, and incrementality tests underneath it to decide where each dollar goes.