In a system design interview, you might be asked, "How would you design a system to store a billion photos?" or "How many servers do you need to handle Twitter's timeline service?"

The secret isn't about giving a perfectly precise answer. It's about demonstrating your intuition for scale.

Estimation is the process of making reasonable, informed guesses to guide your architectural choices. It shows that you understand the trade-offs between different components and can design a system that is both feasible and cost-effective. The goal is not perfect math but a sound thought process.

This cheat sheet provides the real-world reference numbers and quick calculation tricks to anchor your thinking and help you design with confidence.

1. Quick Reference Summary

Let's start with the basics. For simplicity in interviews, always use powers of 10.

2. Latency Numbers (Know These by Heart)