When it comes to calculating an Engagement Rate per post, we should put Audience Quality over its quantity.
At HypeAuditor the report Audience Quality is expressed as the percentage number of real and potentially active followers to the total followers number.
Real People and Influencers are considered quality followers. These types of Instagram users do engage with their followers on a regular basis via comments, likes and post saving. Suspicious accounts and Mass Followers actions on the contrary are identified as non-authentic.
How to calculate an Engagement Rate per post?
ER = (likes count + comments count) / followers count
To get more information about Engagement Rate read our blog post "What is Instagram Engagement Rate and How to Calculate it."