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Google Ads Smart Bidding’s dirty secret: why scaling budget kills your CPA (and what to do about it)
Posted by on January 1, 2026 at 3:16 pmManaging Google Ads for ecommerce brands, I constantly see this frustration: "We're hitting less than $50 CPA target perfectly at $1k/day spend. Let's increase to $2k/day!" Then CPA jumps to $75 ..
Why this happens:
Your original budget let Smart Bidding participate in the absolute best 20% of available auctions – bottom-of-funnel searchers ready to convert right now.
Doubling budget doesn't mean doubling these perfect opportunities. It means:
- Competing in new auctions you weren't winning before (higher CPCs required)
- Reaching less-qualified traffic with identical search terms but different intent
- Learning period where algorithm figures out how to optimize these new segments
The funnel reality:
- Bottom 3% (ready to buy): Easy conversions, your original focus
- Middle funnel (evaluating): Lower conversion rates
- Top funnel (researching): Lowest conversion rates, highest volume
The 20% rule for scaling:
Small accounts: $20 → $40 is fine Large accounts: $1,000 → $1,200 maximum
Wait 1-2 weeks between increases. CPA will spike, then typically recover as algorithm learns.
Has anyone successfully scaled Smart Bidding campaigns without accepting higher CPAs? What's your maximum acceptable efficiency loss for increased volume?
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1 Reply
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Depends on bid strategy; If you bid on tCPA, it should be staying close to the target value
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