ROAS vs MER
Steer the budget with MER and diagnose with ROAS. MER is total store revenue divided by total ad spend for the same period, and it cannot be inflated by attribution because it only uses two numbers you can see in your bank and your billing. Platform ROAS is the return one ad account claims for itself, and it is the right tool for deciding which campaign to change, not how much to spend overall.
The reason is double counting. Run two ad platforms and a fair number of your orders will appear in both dashboards. Add the reported revenue together and it will beat what the store actually took. MER does not have that problem, so it is the number to set a budget by. ROAS still matters inside a platform, because you cannot compare two campaigns on a store wide figure.
Choose
Platform ROAS
when
- You are deciding which of two campaigns in the same account gets more budget
- You want to know whether a new creative or a new product group is carrying its cost
- A single channel is the only paid source you run, so double counting cannot happen
- You are checking whether bidding is hitting the target you gave it
- You need a fast read on a change made yesterday, before the weekly numbers exist
Choose
Blended MER
when
- More than one ad platform is running and both claim the same orders
- You are setting next month's total budget rather than moving money between campaigns
- Reported revenue in the ad accounts no longer matches what the store recorded
- You run influencer, affiliate or offline spend that no pixel will ever see
- You want one number the finance side of the business will accept
| Criterion | Platform ROAS | Blended MER |
|---|---|---|
| Cost | Free in every ad platform, but reported by the party being judged | Free, needs a weekly habit of pulling revenue and spend into one place |
| Setup effort | Correct conversion tracking and one value per order | A spreadsheet or dashboard with total revenue, total spend, week by week |
| Time to signal | Daily, once a campaign has enough conversions | Weekly, because the ratio is noisy over a single day |
| Control | Tells you which campaign to change | Tells you whether the whole spend level is right |
| Best for | Campaign and creative decisions inside one account | Budget decisions across the business |
| What can go wrong | Retargeting looks brilliant by taking credit for orders that were coming anyway | Revenue swings for reasons other than ads, such as a season or an email send |
| How we measure it | Return per campaign compared with your break even, over a full purchase window | Store revenue divided by all marketing spend, tracked weekly against a contribution floor |
When both apply
Use them together with clear jobs. MER sets the ceiling: if blended return drops below the level your margin can carry, spend comes down whatever the ad accounts claim.
Inside that ceiling, ROAS decides where the money sits. Move budget toward the campaigns beating your break even and away from those under it, then check the following week that MER actually moved. If campaign returns improve while MER stays flat, the ad accounts are claiming orders you were going to get anyway, and the honest read is the blended one.
Questions
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