In one line
Spend ÷ installs. CPI (Cost Per Install) is the cost of one app install, and usually the first metric anyone checks in app marketing.
Why it matters
It's usually the first cost metric app marketers look at. But a low CPI isn't automatically good — if the user installs and does nothing else, the install was worthless. CPI only tells you how cheaply you acquired users, not whether they're worth anything; pair it with CPA and LTV to know that.
Comparing channels with it
Before comparing CPI across channels, line up the install definition and the attribution window. Mixing click-through with view-through installs, or platform-reported installs with MMP-reported installs, changes the denominator underneath the comparison. Whether re-installs count is another team decision worth fixing once and writing down.
Why the cheaper CPI is not always the better buy
The arithmetic makes it obvious. Channel A spends $3,000 for 1,000 installs, a $3 CPI. Channel B spends the same $3,000 for 600 installs, a $5 CPI. On installs alone, A wins.
Now add what happens next. If A converts 15% of installs to signup and B converts 33%, that is 150 signups against 200. Per signup, A costs $20 and B costs $15 — the ranking flips. CPI measures the doorway only, which is why budget moves should wait for the downstream numbers.
Go deeper
What CPI, CPA, CPM and CPC each measure, and when to lead with which, is covered in CPI vs CPA vs CPM vs CPC.