Paid media optimisation
Reading CPA and ROAS without fooling yourself
What cost per acquisition and return on ad spend tell you, and what they leave out.
OppRise Editorial · Reviewed 28 September 2026 · 3 min read
The two numbers
Cost per acquisition is spend divided by conversions. Return on ad spend is conversion value divided by spend. Both depend entirely on what you count as a conversion and how it is attributed.
Common traps
The numbers are useful, but easy to misread.
- Comparing channels that use different attribution windows
- Counting low value actions as conversions
- Ignoring margin, so a high ROAS still loses money
- Judging on too few days of data
A better habit
Agree one definition of a conversion, check it matches across platforms and compare against a target based on your margin. Review trends over weeks rather than reacting to single days.
Put this into practice with