Viral Coefficient
AdvancedThe viral coefficient states how many new users each existing user brings in on average, counting only the ones who actually join.
The figure is the product of two simple things: the average number of invites each user sends, and the share of those invites that turn into a real join. A value above one means each generation of users produces a larger one and growth continues without acquisition spend, a state that is rare in practice and usually short lived. Two things must not be dropped: cycle time, the gap between a user joining and sending their invites, matters as much as the coefficient itself, because the same coefficient with a one-week cycle and a three-month cycle produces very different growth curves. A coefficient below one is also not worthless, since it directly lowers the cost of acquiring each customer.
How it is calculated
Viral coefficient = average invites sent per user × conversion rate of an invite into a new user
Both factors must be measured on one defined cohort over a fixed window, otherwise old and new users are averaged together. An invited person who joined but never became active should not enter the numerator. The output is a unitless number and should be reported together with the cycle time.