CAC (customer acquisition cost) is the total cost of acquiring one customer, including everything that goes into acquisition: paid ads, SEO and content, sales people, partnership / influencer payments, attribution and analytics tooling, internal UA team salaries (sometimes). It's a business-level metric, broader than CPI, which only counts ad spend.
Two flavors that matter
- Paid CAC = paid acquisition cost ÷ paid-channel acquired customers. Often ≈ CPI for mobile apps.
- Blended CAC = total acquisition cost (paid + organic-channel investment) ÷ total acquired customers. Includes the SEO content investment, partnership headcount, and other non-ad costs that drive organic acquisition.
Blended CAC is typically lower than paid CAC, because the denominator includes organic users (lower marginal cost). Investors and CFOs usually look at blended; UA teams usually look at paid.
For paid-UA-only mobile apps, CAC ≈ CPI. If your acquisition is 95% paid ads, blended CAC and CPI will be within 5-10% of each other (the gap is the cost of attribution tooling, MMP fees, and UA team overhead).
For apps with strong organic / referral / partnership channels, CAC can be materially lower than CPI. A consumer app that ships 50% of its installs via organic search, App Store featuring, and word-of-mouth can have a blended CAC of $3 against a paid CPI of $8. The ROI ceiling on the paid program rises accordingly — paid users only need to clear the paid CPI; organic users dilute the blended cost.
The danger with blended CAC: it can mask a paid program that's actually losing money. If blended CAC is $4 but paid CAC is $20 and LTV is $15, you're burning cash on every paid install while organic / referral users carry the business. Track both numbers and make paid-program decisions on paid CAC, not blended.