K-factor, also called the viral coefficient, measures how many new users each existing user brings to the product through invitations, sharing, or referrals. It is calculated by multiplying the average number of invitations sent per user by the invitation conversion rate. A K-factor above 1.0 means each user brings in more than one additional user on average, which produces exponential organic growth without any paid acquisition. Even a modest K-factor of 0.3 to 0.5 meaningfully lowers your blended customer acquisition cost.
By Pitchkit · Last Updated October 10, 2026
K-factor equals the average number of invitations each existing user sends, multiplied by the percentage of those invitations that convert into a new active user. The result is the average number of new users each existing user generates through invitations alone, separate from any paid acquisition channel.
A K-factor above 1.0 means the product grows on its own, purely through user-driven invitations, since each cohort of users brings in more than its own size in new users. Even far below that threshold, a K-factor of 0.3 to 0.5 still meaningfully reduces how much you need to spend on paid acquisition to hit the same growth rate.
Each active user sends an average of 5 invitations, and 20% of those invitations convert into a new active user. K-factor equals 5 times 0.2, which is 1.0, the viral breakeven point where the user base could in theory sustain itself through invitations alone.
Most B2B SaaS products run a K-factor of 0.1 to 0.4, since referrals supplement growth rather than driving it entirely. Strong consumer or collaboration products can reach 0.5 to 1.0 or higher, though a sustained K-factor above 1.0 for long periods is rare outside a handful of breakout consumer social apps.
Measuring a one-time spike from a viral moment or press mention instead of sustained, steady-state virality, which overstates how repeatable the growth actually is.
Ignoring invite fatigue, where the same users' invitations convert at a declining rate over time, and ignoring the quality of referred users, since a high K-factor built on low-quality signups that churn quickly is not actually valuable growth.
On the growth slide, show K-factor alongside invite conversion rate and the split between organic and paid new users, for example: K-factor 0.6, with 40% of new users coming from referrals. Showing the trend over several months is more convincing than a single snapshot.
Turn your metrics into a pitch narrative investors can follow.
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