The Diminishing Returns Curve
Why doubling spend never doubles profit
Every Google Ads account has a ceiling. Not a hard stop where ads fail entirely, but a threshold where each additional pound spent returns less than the pound before it. Most brands scale right past this point without noticing.
The Maths Nobody Shows You
Consider an account spending £50k/month with a 5x ROAS. The instinct is obvious: spend £100k, get 10x the revenue. But that's not how Google Ads works. That's not how any paid channel works.
The first £50k captures the highest-intent buyers. The people actively searching for your product, ready to purchase. When you double spend, you're not reaching twice as many of these buyers. You're reaching less-qualified audiences, broader match types, lower-funnel positions.
The Reality of Scaling
Revenue grows. Profit share shrinks. At some point, you're paying more to earn less.
Where the Ceiling Comes From
The diminishing returns curve isn't arbitrary. It's structural. Several forces create the ceiling:
Audience Exhaustion
High-intent searchers are finite. Once you've captured most of them, additional spend reaches people who weren't looking for you. Their conversion rates are lower. Their AOV is lower. Their lifetime value is lower.
Auction Dynamics
Scaling spend means bidding on more competitive auctions. Your cost-per-click rises while conversion intent falls. The maths compounds against you.
Smart Bidding Behaviour
Algorithms optimise for volume when you increase budget. They find more conversions by loosening quality requirements. ROAS targets get hit, but contribution margin erodes.
Finding Your Ceiling
The ceiling isn't a single number. It's a zone where marginal returns start declining faster than you're comfortable with. Finding it requires looking at the right data.
1. Marginal ROAS, Not Blended
Blended ROAS tells you your average. Marginal ROAS tells you what the last £10k spent actually returned. If your blended ROAS is 4.5x but your marginal ROAS on incremental spend is 2.8x, you're already past the ceiling.
Calculate marginal ROAS by comparing performance at different spend levels. Take your current month. Compare it to a lower-spend month. What was the incremental revenue per incremental pound?
2. Contribution Margin Trending
ROAS can look healthy while profit erodes. Track contribution margin (revenue minus COGS minus ad spend) as a percentage over time. If it's declining while spend increases, you've found the ceiling.
"The ceiling isn't where growth stops. It's where growth costs more than it's worth."
3. New Customer Acquisition Cost
Segment your conversions. What does it cost to acquire a genuinely new customer versus remarketing to existing ones? As you scale, new customer CAC rises disproportionately. When it exceeds first-order profit, you're paying for growth that doesn't pay back.
The Ceiling Diagnostic
You're Past Your Ceiling If:
- □Blended ROAS is stable but contribution margin is declining
- □New customer CAC has increased 20%+ in the last 90 days
- □Incremental spend produces less than half the ROAS of core spend
- □Impression share gains require disproportionate budget increases
- □Average CPC is rising faster than average order value
What to Do at the Ceiling
Hitting the ceiling isn't failure. It's information. The question is what you do with it.
Option 1: Optimise Within the Ceiling
Instead of pushing more spend through a constrained channel, extract more value from current spend. This means:
- SKU prioritisation: Assign roles to products and concentrate spend on high-margin items
- Audience refinement: Tighten targeting to focus on highest-value segments
- Bid strategy adjustment: Increase ROAS targets to force efficiency over volume
- Feed optimisation: Improve match rate on high-intent queries
Option 2: Raise the Ceiling
The ceiling isn't fixed permanently. It can be raised through structural changes:
- Market expansion: New geographies or demographics create new high-intent pools
- Product expansion: New categories bring new search demand
- Conversion rate improvement: Site optimisation means more revenue per click, raising viable CPC thresholds
- Brand investment: Higher brand awareness increases direct and branded search, reducing reliance on expensive generic terms
Option 3: Accept the Ceiling
Sometimes the most profitable decision is to stop scaling. If your ceiling is at £80k/month, spending £120k doesn't make you bigger. It makes you less profitable.
The excess budget is better deployed elsewhere: other channels, retention programmes, product development, or simply kept as margin.
The Working Capital Factor
Aggressive scaling has cash flow implications that compound diminishing returns. More spend means more inventory tied up, longer cash conversion cycles, and greater exposure if efficiency drops.
At the ceiling, you're not just earning less per pound spent. You're tying up more capital to earn it. The true return on investment drops faster than ROAS suggests.
Scaling With Eyes Open
The diminishing returns curve isn't an argument against growth. It's an argument against blind growth. Scaling should be a deliberate choice made with full visibility into the trade-offs.
Know your ceiling. Know what it costs to push past it. And know what you're giving up in margin to gain in revenue.
The Bottom Line
Doubling spend never doubles profit because the best customers are already buying. Every increment beyond your ceiling costs more to reach, converts at lower rates, and returns less margin. The smartest operators don't push past the ceiling. They find it, measure it, and make conscious decisions about whether the trade-off is worth it.
Related Reading
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Request a Ceiling AnalysisWritten by Chris Maybury · Published January 2025