Viral Coefficient

The viral coefficient (K-factor) measures how many new users each existing user brings in on average — a K greater than 1 means the user base grows on its own, without paid acquisition.

K > 1 is genuinely rare and hard to sustain for more than short bursts even for products famous for virality — most "viral" growth stories actually run on K somewhat below 1, compounded with paid or organic acquisition on top, rather than pure self-sustaining virality. A K of 0.3, while below the mythical 1.0 threshold, still meaningfully reduces effective CAC by adding free users on top of every paid or organic one acquired.