What is the K-factor and viral coefficient?

Full Explanation

K-factor is calculated as (invites sent per user) × (conversion rate of invites). A K of 1.5 means each user brings 1.5 new users; a K of 2.0 means each user brings 2 new users. With a K > 1.0, the user base grows exponentially; with K < 1.0, the company must rely on paid acquisition to grow. Products like Slack (K~2.5) and Figma (K~2.0) achieve viral adoption through product mechanics that naturally encourage sharing. Most SaaS products have K between 0.5 and 1.3. Products with K < 0.5 require sales-driven growth and are unlikely to achieve viral adoption without significant marketing spend. The viral cycle is measured in days — Slack's cycle is ~15 days, meaning the user base doubles every 2-3 weeks in high-adoption cohorts. For fundraising, demonstrating a K above 0.8 is significant because it signals your growth has an engine beyond paid acquisition. Opagio's calculator includes viral coefficient projections based on your network effects.

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