Calculator · 102
Viral Coefficient Calculator
Measure whether each cohort brings in the next — and decide whether the loop is self-sustaining or needs work.
Viral coefficient (K)
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AverageFormula
K = Invites per user × Invite conversion rate
Understanding the viral coefficient
Reference material — the calculator above stays the primary tool.
What the viral coefficient measures
The viral coefficient, K, is the number of new users each existing user brings in — invites sent multiplied by the share that convert. It is the engine of referral growth: it tells you whether a cohort more than replaces itself through invitations alone.
The threshold is 1.0. Above it, growth is self-sustaining and compounds; below it, referral amplifies acquisition but cannot carry growth on its own.
How to read your result
The result is judged against the 1.0 self-sustaining threshold. The scenario lens then shows how improving invite conversion moves K, pricing the gap toward — or beyond — that threshold.
Below 1.0 — referral assists but does not sustain growth. At 1.0 — each cohort replaces itself. Above 1.0 — growth compounds virally.
What drives K
Two inputs set K, with conversion usually the lever. Treat these as orientation.
| Context | Typical median |
|---|---|
| Invites per user | Prompting & incentives |
| Invite conversion | Offer & friction |
| Cycle time | How fast K compounds |
| Incentive both sides | Lifts conversion |
Levers that raise K
Invite conversion is usually the bigger lever than invite volume: reduce friction to accept, reward both sides, and land invites at moments of high satisfaction. Shortening the invite cycle compounds K faster even at the same value. Model a higher conversion as a scenario above.
K in context
Read this alongside the K-factor and referral growth tools, which the related tools cover. Sustained K above 1.0 is rare; most products use referral to lower blended acquisition cost rather than to grow virally alone.