How do you do back-of-the-envelope capacity estimation in a design interview?
⚡ Short Answer
Derive QPS from DAU × actions/day ÷ 86,400 (and apply a peak factor of ~2–5×). Estimate storage from objects/day × size × retention, and bandwidth from QPS × payload size. Round to powers of ten — the goal is order-of-magnitude sizing to justify architecture (caches, shards, replicas), not precision.
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“How do you do back-of-the-envelope capacity estimation in a design interview?”
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100M DAU, 10 reads/user/day:
reads/day = 1e9
avg QPS = 1e9 / 86,400 ≈ 11.6k
peak QPS ≈ 11.6k × 3 ≈ 35k
2 KB/response → peak BW ≈ 35k × 2KB ≈ 70 MB/s🔥What If?
Think Beyond the Expected
Why bother estimating if the numbers are rough?
The magnitude drives the architecture: 35k QPS says you need caching, multiple app instances, and read replicas/sharding — a single DB won't do. Estimation justifies WHY you add each component and shows the interviewer you can size a system, which is the point (not the exact number).
😂Real World
Every senior system-design interview expects quick capacity math to justify caches, shards, and replicas; the skill is reasonable assumptions + powers-of-ten arithmetic, not precision.
🎯Interviewer's Expectation
Keywords they're listening for:
⚠️Common Mistakes
- ✗Skipping estimation entirely
- ✗Forgetting the peak factor
- ✗Over-precise math instead of magnitudes
✅Best Practices
- ✓State assumptions; round to powers of ten
- ✓Apply a peak/burst multiplier
- ✓Tie the numbers to architecture choices
🔁Follow-up Questions
- 1How does the estimate justify adding a cache or shards?
- 2Read:write ratio — why does it matter?
- 3How do you estimate storage growth over a year?
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I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner. Topic: Scaling (System Design) Interview question: "How do you do back-of-the-envelope capacity estimation in a design interview?" Please: 1. Explain the core idea in simple, plain language, using an everyday analogy. 2. Define any technical terms you use. 3. Walk through one small, concrete example. 4. Finish with a single sentence I can easily remember. Keep the tone friendly and assume I'm new to this topic.
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