K factor (marketing)

In viral marketing, the K-factor can be used to describe the growth rate of websites, apps, or a customer base. The formula is roughly as follows:

i = number of invites sent by each customer  (e.g. if each new customer invites five friends, i = 5) c = percent conversion of each invite  (e.g. if one in five invitees convert to new users, c = .2) k = i ∗ c

This usage is borrowed from the medical field of epidemiology in which a virus having a k-factor of 1 is in a "steady" state of neither growth nor decline, while a k-factor greater than 1 indicates exponential growth and a k-factor less than 1 indicates exponential decline. The k-factor in this context is itself a product of the rates of distribution and infection for an app (or virus). "Distribution" (i) measures the average number of people a host will contact while still infectious, and "infection" (c) measures how likely an average person is to also become infected after contact with a viral host.