Engagement rates for post and for page in the given period
The engagement for a post is the sum of comments, likes and shares that post gets.
The average engagement of a page in a given interval is the average of all the engagement values the posts of that page have in that interval.
The engagement rate for a post answers the question: which percentage of the page's fans engaged with that particular post?
The engagement rate for a page answers the question: which percentage of the page's fans engaged with any post of that page in that time frame?
The formulas for the engagement rates in Facebook are:
The formulas for the engagement rates in Instagram are: