Viral coefficient (also called K-factor) is the average number of new users each existing user brings in. It is calculated as the average number of invitations sent per user multiplied by the conversion rate of those invitations into new active users, used to measure the strength of organic growth loops in product-led, referral-driven, and consumer social businesses. A K-factor above 1 means the user base grows on its own without any acquisition spend; a K-factor of, say, 0.4 means the loop amplifies acquisition but does not replace it.
The formula is straightforward and the inputs are the trap: K = i × c, where i is invitations sent per existing user in a given period and c is the fraction of those invitations that conver...