K-factor is a measure of virality of an app (or a website, or a customer base). Put simply, the K-factor is the total number of new activated users generated by a single existing activated user. The K-factor is a gauge of the health of your mobile app’s referral engine, and of responses to any significant changes you make to your mobile user acquisition strategy.
App developers can calculate their K-factor by multiplying together the following three conversions:
K-factor = (download -> activation)*(activation -> share)*(share -> new viral-driven download)
A note of caution: Be sure to keep time a constant while pulling the conversions; otherwise, there may be some strange, time-sensitive responses.