Definition
The viral coefficient (K-factor) is the average number of new users each user brings in.
Formula
K = invitations sent per user × conversion rate of invitations.
Example
If each user invites 4 people and 10% accept, K = 0.4: every 10 users bring 4 more.
Reading it
Above 1, a product grows on its own; below 1, invitations still lower acquisition cost. Track invitation-sent and invitation-accepted events, with the inviter's ID, to measure it.