napkin math

napkin math (uncountable)

1. (Canada, US) The act of performing quick mathematics often as a means of estimation, as if done on the back of a napkin (or in one's head).
   • 2018, Levi Lusko, I Declare War: Four Keys to Winning the Battle with Yourself‎^[1], page 124:

     Based on my napkin math—and keep in mind I am terrible at math—if you made every conscious choice to do right, your highest potential “life grade” is a D once your bad habits are subtracted.

• back-of-the-envelope