Summing Platform ROAS Overstates It: Use MER to See True Ad Efficiency

Google Ads reports "ROAS 400%," and Meta Ads reports "ROAS 350%." Both look healthy on their own dashboards. But add the two numbers together to judge "overall ad efficiency," and you are looking at more revenue than actually exists — because each platform counts the same purchase as its own win. This guide explains why summing platform ROAS overstates results, what MER (whole-ad efficiency) is, and how to use the two together.

TL;DR#

1. Platform ROAS gets "inflated" per channel

   Each ad platform counts the same purchase as its own result, so summing ROAS exceeds your actual ad-driven revenue

2. MER = Total revenue ÷ Total ad spend

   A single number for whole-business ad efficiency, immune to cross-platform double-counting

3. Use ROAS for per-channel tuning, MER for budget decisions

   Daily optimization → ROAS / Budget allocation and whole-business efficiency → MER

4. MER alone still can't tell new from returning

   A channel with high apparent ROAS is often skewed toward returning customers who would have bought anyway

1. How platform ROAS gets inflated#

Bottom line: each platform counts the same purchase, so summing ROAS exceeds your real revenue.

ROAS (return on ad spend) is a platform's ad-driven revenue divided by its ad spend. The ROAS in your Google Ads dashboard uses "the revenue Google decided was its own win," and Meta's ROAS uses "the revenue Meta decided was its own win."

The problem is that one customer often touches several ads before buying. If someone sees a Meta ad, discovers your product, then searches on Google and buys, both Meta and Google count "this purchase was thanks to my ad." That is double-counting in attribution (how revenue gets assigned to ads).

In numbers, for a store spending ¥1M total per month (Google ¥500K, Meta ¥500K):

Google reported : ROAS 400% → claimed revenue ¥2.0M
Meta reported   : ROAS 350% → claimed revenue ¥1.75M
Sum of platform claims      = ¥3.75M
Actual ad-driven revenue    = ¥3.0M

The claims add up to ¥3.75M, but actual ad-driven revenue is ¥3.0M. The ¥750K difference is double-counting — both platforms counting the same purchases. Simply summing per-platform ROAS overstates your total ad result by 25%.

2. What MER is#

Bottom line: MER is total revenue divided by total ad spend, measuring whole-business efficiency in one number.

MER (Marketing Efficiency Ratio) is your whole-business total revenue divided by total ad spend.

MER (%) = Total revenue ÷ Total ad spend × 100

For the same store, if total revenue — including organic search and direct traffic — is ¥5M against ¥1M of ad spend, MER is 500%. The key is that MER never assigns revenue to a specific platform. Because it makes no assignment, there is no double-counting. You see the plain efficiency of all advertising, with none of the per-platform inflation.

MER has another advantage. Because non-ad revenue — organic search, repeat buyers, word of mouth — also sits in the numerator, it reflects how total revenue moves when you change ad spend. If you double ad spend and MER drops, that added investment is lowering overall efficiency. That shift is hard to spot with ROAS, which only looks inside each channel.

If you want to see profit too, there is a "contribution-margin MER" that uses revenue minus cost of goods and shipping as the numerator. The margin-based view is covered in detail in Gross Margin Breakeven ROAS.

3. When to use ROAS versus MER#

Bottom line: refine inside channels with ROAS, set total spend with MER.

ROAS and MER are not a case of one being correct — they cover different scopes. Relying on only one misleads your decisions.

Where ROAS works

• Per-channel campaign tuning on Google Ads, Meta Ads, and the like
• Comparing which creative or which product's ads are performing
• Daily numbers you can act on right away

Where MER works

• Deciding how much of next month's budget goes to each channel
• Checking whether total business revenue actually grew when you raised ad spend
• Evaluating whole-company ad efficiency including organic and repeat revenue

In practice, refine inside channels with ROAS and decide total spend with MER — a two-layer approach. Chasing per-platform ROAS alone lets you miss the state where "overall efficiency is falling, yet each platform still looks healthy" because of double-counting.

4. What MER still hides about new and returning customers#

Bottom line: MER averages everything, so it erases the gap between channels that win new customers and channels that re-engage returning ones.

MER is convenient, but because it blends everything into one number, something disappears: whether revenue came from new or returning customers.

Ad platforms tend to push delivery toward whoever buys most cheaply. The cheapest buyers are usually "returning customers who would have come back anyway." So channels centered on returning buyers tend to show high apparent ROAS. By contrast, channels that reach new customers who don't know your product yet rarely convert on the spot, so their ROAS looks low.

If you cut a new-customer channel "because its ROAS is low," you close off the entry point for future repeat buyers with your own hands. A search channel that "harvests people who already want to buy" and a social channel that "grows people who will want to buy" should never be judged on the same revenue yardstick.

To avoid the wrong call, you need to split channel revenue by new versus returning. Confirm overall efficiency with MER, but evaluate new-customer contribution separately, by channel and by new-versus-returning. With this two-layer view, you pick the channels truly worth protecting when budgets get tight.

RevenueScope solution

Platform ROAS getting inflated, and MER blending away the new-versus-returning split, share one root: "which channel actually created which revenue" is only visible through each platform's self-report.

RevenueScope does not ask you to enter ad spend. That means it is not a tool that calculates ROAS or MER for you. Instead, using its own tracking, it removes duplicates and shows the revenue that genuinely came from each channel, on one screen. Rather than the inflated figures each platform claims separately, it aligns every channel by where the buyer last came from (Last-touch) — one common yardstick. That ends the cross-platform double-counting.

That said, Last-touch has its own weakness. Because revenue leans toward the channel that made the final push, on its own it understates the contribution of new-customer acquisition. That is exactly why RevenueScope also lets you view channel revenue split by new versus returning. By reading the de-duplicated common yardstick together with the new-versus-returning split, you can separate out "channels with high apparent ROAS that are really skewed toward returning customers."

Concretely, you can compare RPS (revenue per session) by channel, broken down by new and returning. When you calculate MER yourself, RevenueScope keeps the numerator — total revenue — and the per-channel split on one consistent basis. See whole-business efficiency and new-customer contribution from the same starting point, instead of being swayed by each platform's self-report. That is the next step.

FAQ#

Q. Should MER or ROAS be my main KPI?

A. The realistic answer is to give them separate roles. Use ROAS for per-channel optimization, and MER for monthly budget allocation and whole-efficiency decisions. With only one, ROAS alone overstates the whole through double-counting, while MER alone hides the quality of individual channels.

Q. Why shouldn't I sum platform ROAS?

A. Because each platform counts the same purchase as its own result. If one customer touches both a Meta ad and a Google ad before buying, both record it as "my revenue." Summing them exceeds actual ad-driven revenue by exactly that overlap.

Q. Is there a benchmark number for MER?

A. There is no universal target, because it varies widely by industry, gross margin, and ad dependence. What matters is the change, not the absolute value. If MER drops when you raise ad spend, that added investment is lowering overall efficiency. Compare against your own past MER, before and after a change.

Conclusion#

Because each platform double-counts the same purchase, summing per-platform ROAS overstates your total ad result. MER (total revenue ÷ total ad spend) measures whole-business efficiency in one number and is immune to that double-counting. Refining inside channels with ROAS and setting total spend with MER is the basic two-layer approach that keeps ad-budget decisions honest.

MER is not all-powerful, though: it averages away the gap between channels that win new customers and channels that re-engage returning ones. As a first step, calculate MER from your own total revenue and total ad spend, then split channel revenue by new versus returning. "Which channels are truly worth protecting" becomes a decision you can make with concrete numbers.

Related articles#

• ROAS Complete Guide 2026: Formula, Breakeven Point, and 4 Ways to Improve
• ROAS vs ROI: How EC Businesses Can Stop Confusing the Two Metrics
• Gross Margin Breakeven ROAS: Setting Ad Targets That Actually Mean Something

References#

• Google Ads "About attribution models" 2024
• Shopify "Marketing Efficiency Ratio: How To Calculate + Improve MER" 2026
• Shopify "Return on Ad Spend: How To Calculate Your ROAS" 2026