You just landed an on-site interview at your dream company. The schedule lands in your inbox. Most sessions look manageable — coding, behavioural, a hiring manager chat. Then you see it: System Design Interview. Your stomach drops. “Design Twitter.” “Build a URL shortener.” “How would you architect YouTube?” These questions feel impossibly broad. How could anyone design a system that took hundreds of engineers years to build — in 45 minutes?

Imagine your interviewer says: “We’re designing Instagram Stories. How much storage do we need per year?” Most junior engineers freeze. They don’t know where to begin. They feel like they need exact numbers — the real database size, the real compression ratios, the real usage stats. Here’s the secret: you are not supposed to be exact. You are supposed to be directionally correct. Back-of-the-envelope estimation is the skill of producing a reasonable answer in 2–3 minutes using simple math and a handful of memorized numbers.

Designing a system that supports millions of users is challenging — it is a journey that requires continuous refinement and endless improvement. In this post, we build a system that supports a single user and gradually scale it up to serve millions of users. After reading this, you will master a handful of techniques that will help you crack system design interview questions. A journey of a thousand miles begins with a single step.