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What is Google ROAS and Why you Need to be Cautious
We cover Google ROAS, the pitfalls of relying on it alone, and how a blended approach improves marketing decisions.
Many marketers plan their budgets around Google Ads ROAS. It’s an easy metric to track and, on the surface, it looks reliable.
The platform shows strong returns, you’re hitting your targets, and leadership is satisfied. So, you scale your spend.
Then the problems start. Revenue stalls. Costs rise. Margins tighten. Confidence drops. You’re left wondering how your numbers looked right one month and wrong the next.
If that sounds familiar, you’re not alone. In this article, we’ll explain why Google ROAS often gives a false sense of performance, and what you can do to measure it more accurately.
What you’ll find:
💡 Pro Tip
Google ROAS often appears stronger than reality because it overlooks the influence of upper-funnel channels and double-counts conversions across platforms. Marketing mix modelling gives a unified, cross-channel view, revealing true incremental impact, saturation curves, and where real growth potential remains through marginal ROAS insights.
Book a demo of Ruler’s MMM to see how it works
ROAS, or Return on Ad Spend, measures how much revenue you earn for every pound you spend on advertising.
The formula is simple:
Divide your total conversion value by your total ad spend. If you spend £1,000 on ads and generate £4,000 in revenue, your ROAS is 4:1, or 400%.
Ask ten marketers what counts as a good ROAS, and you’ll get ten different answers. A solid range is usually between 2:1 and 4:1, meaning you bring in £2–£4 in revenue for every £1 spent.
But that benchmark varies widely by industry, business model, and profit margin.
It’s easy to see why the metric is so popular. It’s simple to calculate and straightforward to interpret.
A higher number suggests more return for your spend. But that assumption doesn’t always hold true, and that’s where problems start.
We speak to marketers every week who spend 70–80% of their budget on Google Ads.
Almost every time, the story is the same. Revenue stays flat or even declines.
So why does this happen? On paper, Google Ads looks trackable.
People search, click, and convert. The attribution window is long, and with last-click attribution, Google often takes full credit for conversions, even when it’s capturing users who were already influenced elsewhere.
Platforms like Meta, TikTok, etc. work differently. Their ads plant the seed.
People see content, explore communities, read reviews, and talk to friends. Weeks later, they might search your brand on Google or click a paid result before buying.
In your reports, that looks like a Google conversion. In reality, Google is often closing deals that Meta or TikTok started.
The result? Meta and TikTok appear underperforming, so you cut their budgets and push more spend into Google.
Demand falls. Google has fewer warm prospects to capture. And, your efficiency drops.
There’s another issue too, duplication. A customer might click an ad on Meta, then later click one on Google before converting.
Both platforms claim the same sale, while your CRM records only one. Multiply that across hundreds or thousands of conversions, and you’re left with inflated ROAS figures that don’t reflect true performance.
This isn’t about saying Google Ads is wrong, or that you should stop using it.
It’s about understanding its true role in your mix. Google performs best when it’s supported by upstream demand from other channels.
If you judge channels in isolation or rely on limited attribution models, you’ll miss how they interact and end up with inflated metrics that don’t reflect real performance.
To get a full picture, you need a framework that measures how every channel contributes to overall growth.
That’s where Marketing Mix Modelling helps. Unlike click-based systems that work in silos, MMM analyses all marketing inputs together to identify each channel’s incremental impact.
It reveals how close you are to saturation in every channel and calculates marginal ROAS, the return you can expect from your next pound of spend.
This approach shifts planning from educated guesswork to evidence-based decisions. The table below illustrates the difference between platform-reported ROAS and MMM results:
| Channel | Last Click ROAS | Data-driven impressionROAS | MMMROAS | MMM marginal ROAS |
| TikTok Prospecting | 0 | 2.86 | 2.1 | 1.4 |
| Facebook Prospecting | 0 | 2.55 | 4.1 | 2.6 |
| Instagram Prospecting | 0 | 2.35 | 2.3 | 1.2 |
| Google Pmax | 0.6 | 0.87 | 2.3 | 0.9 |
| Google Non Brand | 0.4 | 0.8 | 1.2 | 0.4 |
At first glance, the table reveals a familiar pattern. Google Ads appears to deliver the strongest performance when judged by platform-reported metrics.
But once you introduce marketing mix modelling, a different story emerges, one that challenges the assumption that Google is always the top performer.
Under last-click attribution, Google’s Performance Max and Non-Brand campaigns seem to deliver measurable returns.
In contrast, Meta and TikTok prospecting channels show zero, simply because they rarely capture the final click. This reinforces a bias that pushes spend toward Google, even when it’s not driving new demand.
When you include impression-level and MMM analysis, Meta and TikTok suddenly look far more effective. Facebook Prospecting’s MMM ROAS jumps to 4.1, almost double Google’s 2.3, revealing how upper-funnel activity contributes significant value once you measure beyond clicks.
One of the strengths of MMM is that it can also highlight where future budgets will deliver the highest return.
Google’s marginal ROAS (0.9 for PMax and 0.4 for Non-Brand) suggests those channels are nearing saturation. By comparison Facebook Prospecting’s mROAS of 2.6 indicates room for profitable scaling, while TikTok (1.4) also shows potential for incremental gains.
Google’s ROAS isn’t a bad metric. It’s simple, useful, and still has a place in every advertiser’s toolkit. The metric is still helpful for short-term optimisations. The problem comes when it becomes the only metric you trust, or when you optimise for it without context.
The most effective marketers don’t rely on platform dashboards alone. They use unified measurement to see the full picture of performance.
They know sustainable growth depends on balancing efficiency with reach, short-term wins with long-term acquisition, and revenue with true profitability.
If you’re ready to bring that level of clarity to your own ROAS measurement and future budget planning, book a demo with Ruler to see how unified measurement and MMM can help you do it with confidence.
