Definition
The Viral Coefficient (K-factor) quantifies word-of-mouth growth by measuring how many new users each existing user generates through invitations or sharing. A K-factor above 1.0 means viral growth — each user brings more than one new user, creating exponential growth without paid acquisition. Even a K-factor of 0.3–0.5 significantly reduces customer acquisition cost by supplementing paid channels with organic referral growth.
Formula
How to measure
K = i × c, where i = average invites sent per user and c = conversion rate of invites to signups. Track invite_sent and referred_signup events. Measure per cohort over a defined period (typically 28 days). Segment by invite channel (email, link, social) to optimize the highest-converting channels.
Industry benchmarks
Consumer social apps with strong network effects: 0.5–1.5+. B2B SaaS: 0.1–0.3 is typical. Viral products (Dropbox, WhatsApp historically): 1.0+. Most products realistically aim for 0.3–0.7 as a supplement to other acquisition channels.