There’s no thumb rule or simply speaking there is no standard formula to calculate the engagement rate. In fact there are many ways in which you can calculate the engagement rate and that could depend upon what is it for, whether its a social media or some other platform. One simple way to calculate it is to simply ascertain the total engagement on your website or a particular content and then divide it by total followers, to get the percentage multiply the result by 100.
To see more methods you can check the link below:
https://blog.hootsuite.com/calculate-engagement-rate/