MER vs ROAS: which should you actually measure?
By Tom Goodwin, Founder of GAMEPLAN.
Measure MER for the health of the business and ROAS for the tuning of channels. MER, the Marketing Efficiency Ratio, is total revenue divided by total marketing spend. ROAS, Return on Ad Spend, is the revenue one platform claims divided by that platform’s spend. The mistake I see most often is steering the whole business by platform ROAS, then wondering why a dashboard full of 4x and 5x returns sits above a flat profit line. The platforms are double-counting. MER is not. Use ROAS to allocate inside channels, and use MER to decide how much to spend in total.
Across £20m+ of paid media I have managed, the businesses that grew profitably were the ones watching blended efficiency. The ones that stalled were optimising individual platform ROAS into a wall. Here is how to use each metric correctly.
What is the difference between MER and ROAS?
The definitions are simple. The implications are not.
| MER | ROAS | |
|---|---|---|
| Formula | Total revenue / total marketing spend | Platform revenue / platform spend |
| Scope | Whole business | Single channel or campaign |
| Source of revenue | Your accounts (real money) | Platform attribution (claimed) |
| Can be double-counted | No | Yes, across platforms |
| Best used for | Total budget, business health | In-channel optimisation |
| Affected by cookie loss | Minimally | Significantly |
MER answers one question: for every pound I put into marketing, how many pounds of revenue come back across everything? If you spend £100,000 across Google, Meta, affiliates and email, and you bank £400,000 of revenue, your MER is 4. It does not care which platform claims the sale, which is exactly why it is honest.
ROAS answers a narrower question: within Google Ads, which campaigns return more per pound? That is useful for moving budget from a 2x campaign to a 6x campaign. It is useless for deciding whether to spend £100,000 or £150,000 in total, because every platform reports its own ROAS as if it alone drove the sale.
Why does platform ROAS overstate your real return?
Because attribution is competitive, not cooperative. A customer sees a Meta ad, searches your brand on Google, clicks a retargeting banner, then converts. Meta claims the sale. Google claims the sale. The retargeting platform claims the sale. Add the three reported ROAS figures together and you have credited one £200 order three times.
In 2026 this is worse, not better. Cookie deprecation and consent loss mean platforms increasingly report modelled conversions, statistical guesses about sales they cannot directly observe. Those models are tuned to flatter the platform. So when your Google Ads ROAS says 5x and your Meta ROAS says 4x, the blended reality measured against your actual bank balance might be 2.8x. MER catches that gap immediately, because it divides real revenue by real spend and asks nothing of the platforms.
When should I use ROAS instead of MER?
ROAS still earns its place. Use it for:
- Allocating budget between campaigns inside one platform, where the attribution bias is at least consistent.
- Setting target ROAS values for Smart Bidding, where the algorithm needs a goal expressed in its own terms.
- Spotting a campaign that has genuinely fallen off a cliff versus one that is merely being undercounted.
The rule I work to: ROAS is a steering input for the algorithm and the channel, never the scoreboard for the business. The scoreboard is MER, and behind MER, contribution margin.
What about contribution margin and payback?
MER is the floor, not the ceiling. A 4x MER on a product with 70% gross margin is healthy. A 4x MER on a product with 20% gross margin and high returns may lose money. So the metric I actually steer mature businesses by is contribution margin after marketing, sometimes expressed as a target MER that already accounts for product economics.
| Metric | What it tells you | Who should watch it |
|---|---|---|
| ROAS | Channel efficiency | Media buyers, the algorithm |
| MER | Overall marketing efficiency | Marketing leadership |
| Contribution margin | Profit after marketing and COGS | The board, the founder |
| Payback period | Time to recover acquisition cost | Cash-conscious businesses |
For a subscription or repeat-purchase business, payback period matters more than any single ROAS reading. If you recover acquisition cost in three months and the customer stays 30, a low first-order ROAS is irrelevant. MER and payback together tell that story. Platform ROAS hides it.
How do I set a MER target for my business?
Work backwards from margin. Take your gross margin percentage, decide what proportion of that margin you are willing to spend acquiring revenue, and the implied MER falls out. A business with 60% gross margin that wants to keep half of it as profit can afford a MER around 3.3, meaning it spends roughly £1 of marketing for every £3.30 of revenue. Set that as your line. When blended efficiency sits above it, you have room to spend more. When it dips below, you are buying unprofitable growth and need to find out which channel is leaking.
This is why I rebuild measurement before I touch budgets on any engagement. If you do not know your real MER, you are flying on the platforms’ self-reported numbers, and those numbers are paid to look good.
How do MER and ROAS work together in a healthy setup?
They form a hierarchy. MER and contribution margin sit at the top and decide total spend. Inside that envelope, ROAS and target ROAS bidding allocate the money between and within channels. You scale total spend while MER holds above your target, and you let ROAS rebalance the mix underneath. When MER starts to slip as you scale, that is your saturation signal: the marginal pound is now buying demand you would have captured anyway. That is the moment to hold spend, not the moment a single platform’s ROAS dips.
Most measurement problems I am brought in to fix come down to one error: the business made total-budget decisions using a metric designed for channel decisions. Separate the two and the picture clears immediately.
If your dashboards show strong ROAS but your profit line is flat, your measurement is lying to you, and you are almost certainly making total-spend decisions on double-counted numbers. I run a measurement audit that rebuilds your view around real MER, contribution margin and payback, so every budget decision rests on money you can actually bank. Start here: /work-with-me/paid-media-strategy/measurement-audit.