Viral Coefficient (K-factor)

The viral coefficient, or K-factor, measures how many new users each existing user brings in. You calculate it by multiplying the number of invitations a user sends by the percentage of those invitations that convert. A K-factor above 1 means each user brings more than one new user, so the product grows on its own without paid acquisition. Below 1, virality amplifies your other channels but won't sustain growth alone.

For a founder this is a reality check more than a growth strategy. True viral coefficients above 1 are rare and usually short-lived, so chasing pure virality is a trap. What's useful is tracking K honestly: even a coefficient of 0.4 means every paid or organic user you acquire effectively becomes 1.4, cutting your real acquisition cost. Pair it with cycle time, because a slow viral loop compounds far weaker than a fast one.