Formula
CPI = ad spend ÷ installs from that campaign
Install numbers come from the ad network or from a mobile attribution provider (MMP).
What drives CPI
- Platform and country — iOS installs in the US usually cost more than Android installs in emerging markets.
- Category competition — finance and games are expensive.
- Creative quality and how well the ad matches the store page.
- Store page conversion — a weak page wastes clicks. See app store conversion rate and custom product pages.
- App rating — a 3-star app converts ad traffic worse than a 4.7-star app.
CPI isn't everything
A cheap install that never comes back is worthless. Judge campaigns against lifetime value and day-7 retention, not CPI alone.
Example
A campaign spends $5,000 and brings 2,000 installs: CPI is $2.50. A second campaign's CPI is $4, but its users retain twice as well — it's the better campaign.