Formula
Invitations sent per user x Conversion rate of invitations
How to measure
Track the average number of invites an active user sends and the percentage of those invites that convert into new active users, then multiply the two.
Why track it
A K-factor consistently above 1 means the user base can grow through virality alone, independent of paid acquisition spend.
Example
Users sending an average of 4 invites at a 30% conversion rate produce a K-factor of 1.2.