What ROAS is
ROAS means return on ad spend. Each ad platform works it out for itself: the revenue it gives its own ads credit for, divided by what you spent on that platform.
Platform ROAS = revenue the platform credits to its ads ÷ spend on that platformEach platform decides for itself which sales count.
The key word is “credits”. Meta doesn’t see your WooCommerce orders the way you do. It sees conversions its pixel or its server connection reported, and it decides which of them its ads caused. Google Ads does the same with its own tag and its own rules.
What MER is
MER means marketing efficiency ratio. Some people call it blended ROAS. It ignores what any platform claims. You take your total store revenue for a period and divide it by your total ad spend across every platform.
MER = total store revenue ÷ total ad spend on all platformsUse revenue excluding tax and refunds, from your store, not from the ad platforms.
MER can’t be inflated by double counting. Every dollar of revenue is in it once, and every dollar of ad spend is in it once.
Why platform ROAS over-counts
Each platform reports the sales it touched, by its own rules. A few common habits push the numbers up:
- Two platforms can claim the same sale. A customer clicks a Google ad on Monday and sees a Meta ad on Wednesday, then buys. Both platforms can count that order as theirs.
- View-through conversions. Some platforms count a sale when someone saw an ad and bought later without clicking. The ad may have helped. It may also have been scrolled past.
- Attribution windows overlap. Each platform credits sales for a set number of days after a click or view. Those windows run at the same time, and neither platform knows about the other.
- Customers who would have bought anyway. Returning customers and people who searched your brand name often click an ad on the way in. The platform counts the sale.
Add the claimed revenue from all your platforms and it can easily come to more than your store took in.
A worked example
Here is one month from a sample store running ads on Meta and Google.
| Source | Ad spend | Revenue claimed | ROAS |
|---|---|---|---|
| Meta | $3,000 | $12,000 | 4.0 |
| Google Ads | $1,000 | $9,000 | 9.0 |
| Platforms added up | $4,000 | $21,000 | 5.25 |
| Your store, all sales | $4,000 | $20,000 | MER 5.0 |
The two platforms claim $21,000 in sales. Your store sold $20,000 in total, including customers who came from email, search results and word of mouth. So at least $1,000 of claimed revenue was counted twice, and likely a lot more.
Your MER is $20,000 ÷ $4,000 = 5.0. That number is honest. It still doesn’t tell you if the ads made money. For that you need a line to compare it with.
Break-even MER
Revenue isn’t profit. Before ads, each order has to pay for its products, shipping, payment fees and packaging. What’s left is your contribution margin before ads. Break-even MER is the MER where ads eat all of it.
Break-even MER = 1 ÷ contribution margin before adsExample: orders leave 55% of revenue before ads, so 1 ÷ 0.55 = 1.82.
In the example month, a 55% margin means your $20,000 in sales left $11,000 before ads. You spent $4,000, so $7,000 was left for running costs and profit. At an MER of 1.82 you would have spent about $11,000 on ads to make $20,000, and kept nothing.
How to use it day to day
- MER above break-even: your ads, together, are paying for themselves out of what orders leave. The gap is what’s left for rent, apps and salaries.
- MER close to break-even: you’re buying revenue at cost. Fine for a short test. Not a way to run a store.
- MER below break-even: every extra dollar of ads loses money, whatever the platforms report.
Check MER over a week or a month. One day is too noisy, because sales from today’s ads can land tomorrow.
When platform ROAS is still useful
Platform ROAS is fine for comparing things inside one platform. If two Meta campaigns run under the same attribution rules, the one with 5.0 is probably doing better than the one with 2.0. Use it to decide which ads, audiences or products to push inside Meta or inside Google.
Don’t compare Meta ROAS with Google ROAS. They use different rules, so the numbers don’t mean the same thing. And don’t add them up to judge your total ad budget. That’s MER’s job.
Where MER falls short
- It doesn’t tell you which campaign works. MER is one number for all ads. For campaign choices, go back to the platform.
- It includes organic sales. Email, repeat customers and search results all count. If organic sales grow, MER goes up even if your ads got worse.
- It moves with the season. A holiday rush or a slow summer shifts it without any change in your ads.
So read MER as a trend. If you raise spend by 20% and MER stays above break-even, the extra spend is likely paying off. If MER drops toward break-even, the new money is working less hard than the old money.