Track both, but make decisions on MER. ROAS is revenue a single ad platform attributes to itself, divided by that platform's spend. MER (marketing efficiency ratio) is total store revenue divided by total ad spend across every channel. MER can't be double-counted, so it's the more honest measure of whether marketing is working.
What is MER?
MER, sometimes called blended ROAS, is your total revenue (from Shopify) divided by your total marketing spend (Meta, Google, TikTok and so on) for the same period. If you made $30,000 and spent $10,000 across all ads, your MER is 3.0.
Why ROAS on its own misleads
- Double counting: Meta and Google can both claim the same order.
- Missed sales: someone sees a Meta ad, then searches your brand on Google a week later. Meta may not see it.
- Tracking gaps: privacy changes and ad blockers mean platforms never see every conversion.
When to use each
| Question | Use |
|---|---|
| Is marketing profitable overall? | MER |
| Should I increase total budget? | MER |
| Which campaign or ad is working best? | Platform ROAS and CPA |
| Which creative should I scale? | Platform ROAS and CPA |
How to set a target MER
Your target MER comes from your margin. If contribution margin before ads is 60%, every $1 of ads needs to return more than $1.67 in sales just to break even (1 / 0.6). Add room for profit and fixed costs and many brands land on a target MER of 2.5 to 4.
A simple weekly routine
- Check MER for the week against target.
- If MER is on target, look at platform ROAS to decide where the next dollar goes.
- If MER is off target, check the site and offer before blaming the ads.
Budget and scaling planner
Frequently asked questions
What is a good MER for ecommerce?
It depends on margin, but many profitable ecommerce brands run an MER between 2.5 and 4. Calculate the MER at which you break even, then add a buffer for profit.
Is MER the same as blended ROAS?
Yes. MER and blended ROAS both mean total revenue divided by total ad spend across all channels.
Should I include organic sales in MER?
Yes. MER uses total revenue on purpose, because ads drive sales that platforms can't track, including organic searches and direct visits.
Written by Shimeez Bok, founder of Growth Alchemy, an ecommerce growth studio in Auckland, New Zealand.
