Estimation & Back-of-Envelope Calculations
System design interviews often ask: "How much storage does Twitter need per day?" or "How many servers do we need?"
Useful Numbers to Know
| Fact | Approximate Value |
|---|
| 1 million requests/day | ≈ 12 requests/second |
| 1 billion requests/day | ≈ 12,000 requests/second |
| 1 byte | 1 character |
| 1 KB | Short document |
| 1 MB | Medium image |
| 1 GB | Movie (compressed) |
Estimation Phrases
- "Let me start by estimating daily active users."
- "Assuming each user generates X events per day, that gives us..."
- "Rounding up to be safe, we would need approximately..."
- "That translates to roughly X GB of storage per day."
- "With 10 million DAU and 10 messages per user per day, that is 100 million messages/day, or about 1,200 QPS."