Cost per install (CPI) is a mobile advertising pricing and performance metric that measures how much an advertiser pays to acquire a single app install. It is the dominant efficiency metric for app marketers, who run user-acquisition campaigns specifically to drive downloads of their mobile apps. CPI is calculated by dividing total ad spend by the number of installs generated: spend $10,000 and get 4,000 installs, and your CPI is $2.50.
 

CPI exists because the mobile app economy revolves around acquiring users at scale and at a sustainable cost. App developers and publishers — from games to fintech to e-commerce — compete fiercely for installs, and CPI is the benchmark that tells them whether their acquisition strategy is working. It's the mobile equivalent of CPA, with the "action" being an app install.
 

In a CPI model, advertisers typically pay only when an install is confirmed, which shifts delivery risk toward the ad network or platform and makes the cost highly predictable. Campaigns run across mobile ad networks, DSPs, and in-app inventory, using formats like interstitials, rewarded video, playable ads, and native units that are well suited to driving downloads. Targeting leverages device type, OS, geography, and behavioral signals to reach users most likely to install.
 

CPI varies enormously by factors like platform (iOS installs are usually more expensive than Android), geography (Tier 1 markets cost far more than emerging markets), app category, ad format, and creative quality. A puzzle game in an emerging market might see a CPI under $0.50, while a finance app in the US could run several dollars or more per install.
 

The critical limitation of CPI is that an install is only the beginning of the user relationship. Many installs never lead to engagement, registration, purchases, or retention. This is why sophisticated app marketers look beyond CPI to deeper metrics: cost per registration, cost per purchase, return on ad spend, retention rates, and ultimately customer lifetime value. A low CPI that produces users who immediately uninstall is worse than a higher CPI that delivers engaged, paying customers.
 

Fraud is a major concern in CPI campaigns. Install fraud — through bots, device farms, click injection, and SDK spoofing — can generate fake installs that drain budgets and corrupt data. Mobile measurement partners (MMPs) and anti-fraud tools are essential for verifying that installs are genuine. Used alongside post-install metrics and strong fraud protection, CPI remains the foundational lever for scaling a mobile app — the starting point for turning ad spend into a growing, valuable user base.