I am getting rejection on my 13.5MB attachment even though size limit is 15MB. So, I want to know the exact policy by Gmail, as in how do they calculate the size of an email which contains attachment?
The 15MB limit is for the entire message, not just attachments.
To check the size of a message, open it, and select Show Original from the menu in the upper right. This opens the entire message in a new tab/window. Save this to a plain text file and the size of that file is the total size of the message.
Tl;dr: If Gmail says your message is too big, you'll have to make it smaller. Try sharing the file via Drive or Dropbox and emailing a link.