A practical framework for using MER, platform ROAS, new-customer CAC, and contribution margin to make defensible e-commerce budget decisions.
MER and platform ROAS answer different questions. Platform ROAS helps an ad system and its operator decide where to route spend inside an account. MER shows whether total business revenue is keeping pace with total paid-media spend. Neither metric proves incremental profit. A useful operating system pairs MER with new-customer CAC and contribution margin, then uses platform ROAS as a diagnostic—not as the company scoreboard.
Platform ROAS is the conversion value credited to a platform divided by spend on that platform. The word credited matters. Google Ads explains that its attribution model determines how conversion credit is distributed and that the selected model also affects conversion-based bidding. In other words, platform ROAS is part measurement and part optimization input—not a neutral copy of the finance ledger.
MER, or marketing efficiency ratio, is often used in e-commerce to mean total revenue divided by total paid-media spend:
MER = total business revenue ÷ total paid-media spend
Teams do not always use the term consistently. Some include agency fees, creators, affiliates, or other marketing costs in the denominator; others include only media. Name the version in the dashboard. This guide uses paid-media MER.
The numerator needs a contract too. Shopify distinguishes gross sales, net sales, and total sales, and its total-sales calculation can include taxes, duties, shipping, and fees. For a management dashboard, select a revenue definition that matches the decision—often net sales after discounts and reversals—and keep it consistent across periods.
A shopper may see a Meta ad, search the brand on Google, open an email, and buy. Each reporting system observes a different portion of that path and applies its own rules.
Google Analytics defines attribution as assigning credit to touchpoints and offers several reporting models. It also notes that modeled key-event data can be updated after the conversion is recorded. Shopify's marketing reports warn that sales attributed to marketing can differ from sales in other reports. These are expected consequences of different scopes, windows, identity signals, and models—not automatically evidence that one system is broken.
That creates two common mistakes:
Adding Meta-attributed revenue and Google-attributed revenue as if the totals were mutually exclusive.