The K-factor is a growth metrics borrowed from epidemiology that quantifies how many new users or viewers each existing user generates — essentially measuring the viral coefficient of content or a platform. A K-factor above 1 means each viewer brings in more than one new viewer on average, resulting in exponential growth hacking. On YouTube, a high K-factor is driven by shares, recommendations, and word-of-mouth marketing — indicating that content has strong virality potential. It is closely related to organic reach and is a key indicator of whether a channel growth strategy is generating self-sustaining momentum.