K-Factor
The viral growth rate calculated as invitations per user multiplied by conversion rate per invitation.
Definition
K-factor is the mathematical expression of virality. K = i x c, where i = average invitations sent per user and c = conversion rate of those invitations. K > 1 produces exponential growth; K < 1 produces decaying growth that eventually plateaus. Most consumer apps have a K-factor between 0.15-0.7. Truly viral products (K > 1) are rare and typically involve communication or collaboration tools where sharing is inherent to the product.
K-factor should be measured in conjunction with viral cycle time (the time from user signup to their invitees signing up). Shorter cycles compound faster.
Why it matters for founders
K-factor quantifies whether your product can grow organically. Even modest improvements in K-factor have outsized effects on long-term growth. Increasing K from 0.3 to 0.6 doesn't just double viral growth - it reduces your net acquisition cost by half.
Example
WhatsApp achieved a K-factor well above 1.0 because messaging requires recipients. Each user invited multiple contacts, and the conversion rate was high because receiving a message is inherently compelling. This viral loop helped WhatsApp reach 450M users with only 55 employees.
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Related terms
Viral Coefficient
The number of new users each existing user generates through referrals or sharing.
Referral Program
A structured system that rewards existing users for bringing in new customers.
Customer Acquisition Cost (CAC)
The total cost to acquire one new customer.
Engagement Rate
A measure of how actively users interact with your product or content.