Why Google Ads Returns Can Diminish at Scale
Increasing spend can reach buyers with different intent and economics. A blended return may hide what the next pound of advertising actually contributes.
Diminishing returns are a commercial risk to investigate, not a fixed threshold every brand will encounter at the same spend level.
Blended metrics average together different products and kinds of demand. That average cannot establish the return on additional spend.
Understanding Marginal ROAS
Blended ROAS is an average. It tells you what your total spend achieved in total revenue. What it does not tell you is the return on your next pound of spend. This is marginal ROAS, and it is the only metric that matters when you are deciding whether to scale.
Illustrative numbers only, not a benchmark or client result.
The Diminishing Returns Curve
High-intent brand and product searches
Category searches, competitor conquesting
Broader audiences, demand generation
Creating demand from scratch
The illustrative curve shows why you should distinguish existing demand from new acquisition. It does not predict how an individual account will behave.
The next tranche has to work harder. You are now bidding on category terms, trying to intercept shoppers who are considering your competitors. The conversion rate drops. The ROAS drops. But your blended number stays acceptable because it is averaged with the efficient spend.
The Blended Average Lie
As an illustrative example, a 4.0 blended ROAS at £50k/month could mean: £20k at 6.0 ROAS (harvesting existing demand) + £30k at 2.7 ROAS (creating new demand). The average looks fine. But that £30k in demand creation might be losing money on every sale when you factor in true unit economics.
Why Your Agency Will Not Tell You This
Agencies have an incentive to grow your spend. Not because they are malicious, but because that is how their model works. When you ask "can we scale?", the answer is almost always "yes, here's how." The answer is rarely "no, you've captured most of the profitable demand."
The uncomfortable truth is that many brands are already at or near their efficiency ceiling. Pushing harder will grow revenue but shrink profit. This is not a failure of the agency. It is a feature of how Google Ads works.
"Harvesting" Campaigns
- • Brand search terms
- • High-intent product searches
- • Remarketing to warm audiences
- • Target: 5.0+ ROAS
- • Goal: Maximum efficiency
"Conquesting" Campaigns
- • Category generic terms
- • Competitor brand terms
- • Cold prospecting audiences
- • Target: 2.0-3.0 ROAS
- • Goal: New customer acquisition
The Solution: Separate the Budgets and Expectations
You cannot use the same structure for harvesting and conquesting. You cannot apply the same ROAS target across fundamentally different campaign types. And you cannot make sensible decisions using blended metrics that average them together.
1. Segment by Intent Level
Create distinct campaign types for high-intent (brand, product-specific) versus low-intent (category, awareness) traffic. Apply different bidding strategies and ROAS targets to each.
2. Set Different Profitability Thresholds
Your harvesting campaigns should run at high ROAS with unlimited budget. Your conquesting campaigns need explicit budget caps and lower ROAS targets tied to new customer acquisition value.
3. Measure Marginal Returns Explicitly
Track what happens when you increase spend by 20%. Does revenue increase by 20%? By 10%? By 5%? This tells you whether you are approaching your efficiency ceiling.
4. Accept Lower ROAS for Expansion
If your true customer lifetime value justifies it, a 2.0 ROAS on new customer acquisition can be highly profitable. But you need to know this explicitly, not have it hidden in a blended average.
When to Push Through the Wall
Not every brand should increase spend. For some, the better choice may be to serve existing demand efficiently and invest the would-be ad spend elsewhere: product development, international expansion, or retained earnings.
But if you are going to push, you need to do it with open eyes:
- Accept that blended ROAS will decline, potentially significantly
- Understand exactly where your profitability floor sits for each campaign type
- Have clear customer lifetime value data to justify lower first-order returns
- Structure your account to surface marginal performance, not hide it in averages
Diminishing returns are not tied to a universal spending threshold. There is a finite pool of high-intent buyers for any product category. Once you have captured most of them, incremental spend has to work harder. The brands that scale profitably are the ones that understand this and plan for it, rather than discovering it through collapsing margins.
The Question to Ask
Before your next budget increase, ask this: what was the marginal ROAS on the last 20% of spend we added?
If your agency cannot answer that question, you are flying blind. If the answer is lower than your profitability threshold, you are losing money to grow revenue. And if nobody has ever asked the question before, you need a partner who thinks about your business, not just your campaigns.
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