K Factor is a metric that measures the virality of a mobile app. It measures how many new users an app developer acquires from an existing user’s referral. Calculating the users that existing users bring to an app helps app developers to understand the effectiveness of their user acquisition campaigns. 

If K-Factor is higher for one ad network than another, it means that ad network brings much more engaging and quality users. This is why K factor is preferred to be calculated during a user acquisition campaign. It requires a healthy and powerful referral mechanism inside the app to calculate the K factor concisely. 

As a starting point, creating an ad campaign where an app has not any users is a best practice to calculate K factor effectively. If a mobile app has a K factor of 1.0, it can be said that the app goes viral.