What is K-factor?
K-factor, also called the viral coefficient, is the number of new users each existing user generates through invitations. It is the math behind whether a product grows on its own or needs paid fuel.
A K-factor of 1 is the dividing line. Above it, each user brings more than one new user and growth compounds; below it, referrals amplify other channels but do not sustain growth alone.
The formula.
K-factor = invitations sent per user times the conversion rate of those invitations. If each user sends 5 invites and 20% convert, K is 5 times 0.20, which is 1.0.
Both inputs are levers. You can lift K by prompting more invitations or by making the invitation itself convert better, and the second is usually cheaper.
A worked example.
Each user sends 5 invitations, and 20% of them convert. K-factor is 5 times 0.20, which is 1.0, the threshold where growth becomes self-sustaining.
What is a good K-factor?
Above 1 is genuinely viral and rare; most successful products run well below it and rely on virality to amplify paid and organic, not replace them. A K of 0.3 still meaningfully lowers blended acquisition cost.
Virality and search both lower the cost of acquisition, and they compound. A product with even modest virality plus strong organic spends less per customer, which feeds your customer acquisition cost.
How to improve it.
Build sharing into the moments of value, make the invitation easy and worth accepting, and shorten the loop between invite and signup. Every step removed lifts the conversion half of the equation.
The other side is reach. See how we grow the user base that powers the loop.