Cost per acquisition = Total marketing spend / Number of conversions. If you spend $5,000 on Google Ads and get 50 customers, your CPA is $100. Simple math, but the implications are enormous because CPA determines whether your growth is profitable or just expensive.
Here's what most marketers miss: improving your landing page conversion rate is the fastest way to lower CPA. Doubling your conversion rate literally halves your CPA — same traffic, same ad spend, twice the customers. No amount of bid optimization or audience targeting competes with that leverage.
CPA and landing page quality
Google Ads directly ties landing page quality to your costs through Quality Score. A landing page that loads slowly, has poor mobile experience, or doesn't match ad intent will have a lower Quality Score, which means higher cost-per-click, which means higher CPA. Google is literally charging you more for having a bad landing page.
To measure CPA properly, segment by channel and campaign. Your branded search CPA and your cold prospecting CPA are completely different metrics. Blending them gives you a number that's wrong for both. And always compare CPA to customer lifetime value — a $200 CPA is catastrophic for a $50 product but perfectly healthy for a customer worth $2,000.