# Back of envelope calculations — Cheat Sheet

## Useful numbers for empowering System Design.

**Note: Please make sure to do your own research and don’t rely on these numbers. These numbers are based on my own research and will be outdated by the time you read this article.**

Back of envelope calculations is a technique used within software engineering to determine how a system should be designed. The method is most famous from big tech companies and is often expected in system design interviews. The thought is that you should first calculate some rough numbers so that it can drive decisions in designing possible solutions.

The following article lists various numbers at the time I did the research that I found useful for systems design, but is no longer being updated.

*Note: All the numbers in this post are heavily rounded as their purpose is to give a rough guide for design decisions in the moment. You should always do more precise calculations before starting on a project/feature.*

# Useful Calculations

**x Million users * y KB = xy GB**

example: 1M users * a documents of 100KB per day = 100GB per day.

**x Million users * y MB = xy TB**

example: 200M users * a short video of 2MB per day = 400TB per day.