Definition
CPI, cost per install, is the price paid for one mobile app install, either as a pricing model (the advertiser is billed per install) or as a derived metric (total spend divided by installs). It is the standard unit for mobile user-acquisition volume, and campaigns bought on other models — cost per action, or optimised toward a post-install event — still report an effective CPI as a reference.
CPI is a shallow metric on its own because install quality varies enormously by source. An incentivised install, where the user is rewarded for installing, costs little and is usually worthless downstream.
A bot install costs even less and is worth nothing. A genuine install from a well-matched audience costs more and may be worth many times its price.
So a low CPI is only good news if the installs behind it register, deposit and retain, which is why buyers optimise toward post-install events rather than raw installs wherever the platform allows it.
In context
For iGaming, where the app-store gambling category is restricted in many markets, CPI campaigns run in the markets where real-money apps are permitted, or via web-to-app funnels that produce an install as an intermediate step. The chain of metrics that matters is install → registration → verified → first deposit → retained, with a rate at each step, and the real acquisition cost is cost per qualified FTD, not CPI.
Attribution complicates CPI reporting on iOS, where App Tracking Transparency prevents deterministic linking of most installs to a source. Post-ATT, a large share of iOS installs cannot be tied to a campaign, so iOS CPI and everything derived from it is partly modelled, and buyers weight Android and web funnels — where the signal is fuller — more heavily when allocating budget.
A CPI that looks efficient on iOS may be masking installs the model cannot see or value.
Worked example
A CPI campaign buys Android installs at $1.20. Downstream, 12% register and 3% make a first deposit, so the real cost per FTD is $40. A second source at a $2.00 CPI converts at 6% to deposit, for a $33 cost per FTD — the more expensive installs are the cheaper players, and the buyer shifts budget accordingly.
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