Cost Per Acquisition (CPA)
CPA measures the average cost to acquire a user who completes a specific in-app action, providing a deeper efficiency metric than CPI.
Definition
Cost per acquisition (CPA) measures how much advertising spend is required to drive one user to complete a defined conversion event, typically an in-app purchase, subscription start, tutorial completion, or registration. Unlike CPI which counts any install, CPA measures quality acquisition: users who took the action you actually care about.
CPA is the natural next metric after CPI in the UA efficiency hierarchy. A campaign may achieve excellent CPI but poor CPA if the installs it drives don't engage or convert in-app. The gap between CPI and CPA reveals your install-to-action conversion rate and highlights where funnel optimization is needed, in the creative (pre-install) or the onboarding experience (post-install).
Platforms like Meta allow bid optimization directly toward CPA events through their conversion campaign objectives, meaning advertisers can tell the algorithm to find users likely to complete specific in-app events, not just install.
Why it matters
CPA aligns creative performance measurement with revenue outcomes. A creative that drives cheap installs from low-intent users will have low CPI but high CPA. The installs don't convert to actions. Evaluating creatives at the CPA level (not just CPI) tells you whether you're acquiring users who actually do something valuable inside the app.
Formula
CPA = Total Ad Spend ÷ Number of Defined Conversions
Example
In practice
A fitness app targets subscription starts as its CPA event. Two creatives achieve the same CPI of $3.50, but Creative A drives $12 CPA (29% install-to-subscribe rate) while Creative B drives $28 CPA (12.5% rate), making Creative A clearly superior despite equal install costs.
Frequently asked questions
How is CPA different from CPI?
CPI measures cost per install (the user downloaded the app). CPA measures cost per action (the user completed a meaningful in-app event like a purchase or subscription). CPA is a higher-quality, further-down-funnel metric.
Track CPA by creative with Segwise
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