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.