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What ROAS measures
ROAS (return on ad spend) is the revenue Meta credits to your ads, divided by what you spent on Meta.
The key word is credits. With Meta’s default attribution setting, a purchase counts as an ad sale if it happens within 7 days of someone clicking your ad, or 1 day of them viewing or engaging with it without clicking. Your store platform, meanwhile, credits the last link a shopper used, or nothing at all.
So ROAS is Meta’s account of what Meta did. It’s useful, but it isn’t neutral.
What MER measures
MER (marketing efficiency ratio) is your total store sales ÷ total ad spend, for the same period. The sales figure comes from Shopify, WooCommerce or whatever runs your store, not from Meta. The spend figure is every paid channel you run.
MER doesn’t care which ad got the credit. It asks one question: for every dollar of ads, how many dollars of sales did the store take?
Why they disagree
- Meta counts sales you’d have made anyway. Broad and Advantage+ campaigns can reach your existing customers, email subscribers and people already on their way to buy. Retargeting is the strongest case: many cart abandoners come back on their own.
- Meta misses some sales. Ad blockers and privacy settings hide some shoppers, even with the Conversions API on, and Meta only credits sales inside its attribution window.
- Refunds stay in. Meta counts a purchase when the order is placed. None of the main store platforms sends refunds back to Meta.
- Doubled tracking. A pixel installed twice counts one sale as two.
A small gap is normal. A growing one is a signal.
Which number to use for what
| The question | The number |
|---|---|
| Which ad is working best? | Meta’s ROAS or cost per purchase, at ad level |
| Is the spend paying for itself? | MER, against your break-even ROAS |
| Are the ads adding sales? | Store sales against your baseline |
| Are they finding new customers? | New-customer orders in your store’s reports |
Use Meta to compare ads with each other. Use your store to decide whether the spend is worth it.
Set your MER target from your margin
Your MER target is the same as your target ROAS, and its floor is your break-even ROAS: 1 ÷ your contribution margin. Below that, the store loses money after ads. The break-even ROAS calculator gives you both numbers from your order value, margin, shipping and payment fees. Break-even ROAS, explained shows the working.
The lift check
MER has a blind spot of its own: it includes sales that would have happened without ads. So before your first ad goes live, write down your baseline from your store platform:
- Average weekly revenue for the last 6 weeks.
- Average weekly orders for the same period.
- The share of orders from returning customers, if your platform reports it.
Each week, take store sales minus that baseline. To pay for themselves, your ads need to add at least weekly ad spend × break-even ROAS above it.
Five minutes a week
Do it at the same time each week, ideally Monday or Tuesday morning, so it covers a full weekend of sales.
- From your store, last 7 days: total sales (net of refunds), orders, and how many came from new customers.
- From Ads Manager, last 7 days: amount spent, purchases and cost per purchase.
- Work out MER: store sales ÷ amount spent. Compare it with your target and your break-even.
- Do the lift check against your baseline.
Scale only after 7 days under your target cost per purchase and MER at or above target: raise the daily budget by 20%, once a week at most. If MER falls below target after a raise, go back to the last budget that held.
What a ROAS–MER gap looks like
In the first result below, a client’s store saw Meta’s reported ROAS fall from 4.93x to 4.09x while whole-store revenue rose from A$59,000 to A$65,940 and MER from 3.51x to 3.93x, on unchanged total paid spend. Judged on ROAS alone, it looked worse. Judged on the store, it was better. In the second, recommendations lifted Meta spend and purchases, but MER fell from 5.22x to 3.87x while store revenue barely moved.
Neither proves Meta was wrong. ROAS and MER measure different things. That’s the point of watching both.
Not sure your tracking is counting properly? The free 3-minute quiz checks it with the other usual causes. Plans for online fashion, beauty and skincare, jewellery and accessories and home and lifestyle stores build this scorecard in.