What are Back-of-the-Envelope Estimations?

Back of the envelope estimations in system design interviews are like quick, rough calculations you might do on a napkin during lunch - they're not detailed or exact, but give you a good ballpark figure. These rough calculations help you quickly assess the feasibility of a proposed solution, estimate its performance, and identify potential bottlenecks.

Purpose

Back-of-the-envelope estimation is a technique used to quickly approximate values and make rough calculations using simple arithmetic and basic assumptions