Viral Coefficient
Also called K-Factor and K Factor
How many new users each existing user brings on average, also called the k-factor. Above 1 means self-sustaining, exponential growth from referrals alone.
K-factor = invites sent per user x conversion rate of those invites
Why it matters
A viral coefficient above 1 is rare and powerful, it means growth compounds without paid spend. Even below 1, a higher k-factor lowers your effective acquisition cost, so it is worth engineering into the product.
For example
Each user invites five friends and 20 percent join, a viral coefficient of 1.0, meaning growth can sustain itself without paid spend.
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Related terms
Also in Starting Up
See how founders actually handle this on Money, pricing and metrics, part of the Starting Up hub.
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