Viral Coefficient (K-Factor)

The viral coefficient, or K-factor, measures how many new users each existing user brings in through invitations or shares. Multiply the average number of invites a user sends by the conversion rate of those invites: if 100 users each send 4 invites that convert at 25%, K = 1.0, meaning every user recruits one more and growth is self-sustaining. A K above 1 produces exponential growth; below 1, referrals amplify other channels but don't stand alone. For SaaS builders it quantifies whether a referral or sharing loop is actually working. Why it matters: teams often assume "going viral" is luck, but K is an engineerable number driven by invite prompts, incentives, and cycle time. Practical note: most products never exceed K=1, and that's fine — a K of 0.4 still cuts effective acquisition cost sharply. Optimize invite placement, reduce friction to accept, and shorten the loop's cycle time.