MER

MER is a single number that tells you how much revenue your whole store made for every pound or dollar you put into marketing. It ignores which channel did what. You take everything the store sold in a period, divide it by everything you spent on advertising in that period, and that ratio is your MER.

MER is not a channel metric, and it is not the same as ROAS. ROAS is reported by an ad platform and counts only the revenue that platform claims credit for. MER counts all revenue, including repeat customers, email, organic search and direct traffic, against all spend. People confuse the two constantly.

Divide total revenue for the period by total marketing spend for the same period. The honest denominator includes every marketing cost: ad spend across all platforms, agency or freelancer fees, influencer payments, tool subscriptions. The flattering denominator is paid ad spend only, which makes the ratio look higher without changing anything about the business. Pick one, write it down, and keep using it, because MER is only useful when this month compares cleanly to last month.

ROAS is platform-reported and channel-specific, where MER is blended across everything. Break-Even ROAS tells you the ratio you need to cover costs, and you can calculate the same threshold for MER. CAC looks at cost per new customer rather than revenue per pound spent. Incrementality asks whether the spend caused the revenue at all, which MER cannot answer.