The basic assumption for viral growth ('K') is that every new user of a product becomes an evangelizer for that product and will convert others to start using that product as well.

K = invites * conversion rate

There are two variables that allow you to create a model for viral growth:

1. Viral Coefficient: the total number of new users generated by an existing user
2. Viral Cycle Time: the amount of time it takes before all these new users have been generated (time for the viral coefficient to repeat itself)

Use the "K Factor" sheet of the workbook to help calculate this.