The short version
MER, or marketing efficiency ratio, is total revenue divided by total marketing spend.
MER = total revenue ÷ total marketing spend
Break-even MER = 1 ÷ contribution margin
$200,000 of revenue on $50,000 of marketing is a MER of 4.0. At a 40% margin, break-even is 2.5, so this month's marketing more than paid for itself.
Why MER and not ROAS?
Every platform takes credit for the same sale, so platform ROAS numbers add up to more than you sold. MER uses your store's revenue and your total spend, so nothing gets counted twice.
ROAS picks the ads. MER tells you whether the ads are paying the bills.
I go through the bank statement test, aMER and when to stop scaling in what is MER in marketing.
aMER: the stricter version
Returning customers count in MER even if ads didn't bring them back. Acquisition MER only counts revenue from new customers:
aMER = new customer revenue ÷ total marketing spend
If MER holds steady while aMER falls, your spend is taking credit for customers you already had.
Which number should I trust?
MER for the whole budget. Platform ROAS for choosing between ads and campaigns inside it. If you want to judge a single campaign, use the ROAS calculator. For your break-even per order, use the break-even ROAS calculator.
Pool rule 009Report decisions, not dashboards.
Quick answers
What is a good MER? One comfortably above your break-even MER. At a 40% margin, anything well above 2.5.
Is MER the same as blended ROAS? Yes. Both divide all revenue by all marketing spend.
Why do some tools show MER as a percentage? They flip it: spend divided by revenue. A MER of 4.0 shows as 25%. Check which way your tool does it.
Should agency fees count as marketing spend? Your call. Pick one definition and keep it, so month-to-month comparisons stay honest.