Engagement Rate illustrates the level of engagement with content published by a given page or profile. It is calculated using the sum of interactions divided by the number of fans or followers. It is then multiplied by one hundred to produce the result in a friendlier format (fewer zeroes).
ER for a post is calculated as the sum of interactions for this given post and divided by the number of fans/followers on the day the post was published.
Daily ER for a page or profile is calculated as the sum of interactions to all posts published on one given day and divided by the number of fans/followers on that day.
ER for a period longer than one day (e.g. week or month) is calculated as an average of daily ERs for the given period of time.