Q:

How does Gmail calculate the size of a message which contains an attachment? 

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? 

A:

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. 