Viral Coefficient
Viral Coefficient (K-factor)
In one line
Viral coefficient (K-factor) measures how many new users one existing user generates on average — once it crosses 1.0, the product grows on its own without ads.
Going deeper
K-factor is invites per user multiplied by invite acceptance rate. If a user invites 5 people on average and 20% sign up, K equals 1.0. Cross 1.0 and the product enters true viral growth — every new user generates more than one additional user.
Genuine K above 1 is rarer than the literature suggests. Early Hotmail, early Dropbox and pandemic-era Zoom are the canonical cases — all of them with very specific product-environment fit. Most SaaS lives somewhere between 0.3 and 0.7, which still meaningfully cuts ad dependence and is worth the work.
Chasing K alone backfires. Forced invite flows pump the number short term but burn user trust fast. Durable K comes from a product experience users actually want to share, not from a manipulative onboarding step.
Related terms
Growth Loop
A growth loop is a self-reinforcing cycle in which existing users' actions generate new users — the modern alternative to the linear funnel for sustainable growth.
MarketingPLG
Product-led growth (PLG) is a model where the product itself drives acquisition, conversion and expansion — making it the default growth strategy for many modern SaaS businesses.
MarketingGrowth Hacking
Growth hacking is the discipline that combines data, rapid experimentation and product changes to find scalable levers for user and revenue growth.
MarketingWord-of-Mouth Marketing
Word-of-mouth marketing engineers the conditions for customers to recommend the brand naturally — turning the most trusted channel of all, peer recommendation, into a marketing engine.
MarketingCustomer Advocacy
Customer advocacy turns satisfied customers into active brand advocates whose voice — through reviews, referrals and community — becomes a primary acquisition channel.