Definition
The canonical answer.
Cost per install (CPI) is ad spend divided by the installs it produced. It is a price, not a verdict: a cheap install that never monetizes is expensive, and a costly install with strong lifetime value is cheap. CPI belongs next to cohort revenue, never alone.
Worked example
In practice.
Two campaigns run: one buys installs at half the CPI of the other, but its cohorts barely register revenue while the pricier channel’s users subscribe. Judged on CPI the wrong campaign wins; judged on cost per paying cohort the budget moves the right way.
Where this work happens: User Acquisition
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