## The ARPU > CPI Equation

The fundamental law of mobile applications is that your Average Revenue Per User (ARPU) must exceed your Cost Per Install (CPI). However, many developers look at ARPU only for users who stick around, drastically overestimating their financial health.

### FAQ

**Q: Why does Day-30 Retention matter here?**
A: If it costs you $3 to get a user to download your app (CPI), and a subscribing user generates $22 for you... you seem highly profitable. But if 88% of people delete your app by Day 30, it means you actually spent $33 to acquire that single retained user ($3 * 11 installs). You are losing $11 per retained user. You must use 'Blended LTV' across all installs.