We have two categories for reach - one for social media and another one for webpages.
For social media, the algorithm takes into consideration the number of followers on a page/ profile. Then it takes in calculation share, comments, and likes - each having its own value - from the posts (share being stronger than like since share means more people will potentially see and like a post on a different profile).
For webpages, the reach is calculated as following - from global pages that monitor visits per day on a certain webpage we take the average number of visits. Then it measures whether the news was on the main page, subdomain, or subcategory, whether the keyword is mentioned in the title or somewhere on the side.