Viral Coefficient
The number of new users each existing user generates through referrals or sharing.
Definition
Viral coefficient (K-factor) = number of invitations sent per user x conversion rate of invitations. A viral coefficient above 1.0 means exponential growth: each user brings in more than one new user. Below 1.0, growth is sub-viral and requires paid acquisition to supplement. Even a K-factor of 0.5 is valuable because it means organic sharing covers half your acquisition cost.
Viral growth also depends on cycle time - how quickly the viral loop completes. A K-factor of 1.2 with a 1-day cycle grows much faster than K=1.5 with a 30-day cycle.
Why it matters for founders
Viral growth is the most capital-efficient way to scale. If your viral coefficient exceeds 1.0, you can grow without spending on marketing. Even sub-viral coefficients dramatically reduce CAC.
Example
Hotmail added "Get your free email at Hotmail" to every outgoing email. Each user effectively sent invitations to everyone they emailed. This produced a viral coefficient above 1.0, helping Hotmail grow from 0 to 12 million users in 18 months with zero ad spend.
How Foundra helps
Foundra's Launch Assets card helps you design viral mechanics into your product from the start, whether through referral incentives or inherently shareable features.
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Related terms
K-Factor
The viral growth rate calculated as invitations per user multiplied by conversion rate per invitation.
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.
Product-Led Growth (PLG)
A growth model where the product itself drives acquisition, retention, and expansion.