Back-of-Envelope Math
1 min read
Estimate QPS, storage, and bandwidth from DAU using simple arithmetic.
Convert DAU into QPS, storage, and bandwidth with simple arithmetic. The process matters more than the exact numbers.
How It Works
Back-of-envelope calculations convert user counts into infrastructure numbers. The formula chain: DAU -> daily actions per user -> total daily actions -> QPS (/ 86400) -> peak QPS (x 2-5x). For storage: actions per day x bytes per action x retention period. Key numbers: 86,400 seconds/day, 1 char = 1 byte, 1 image ~200KB. In interviews, write the math out — the process matters more than exact numbers.
Real-World Example
For a URL shortener with 100M DAU: 10M writes/day = ~120 writes/sec, 100M reads/day = ~1200 reads/sec. Peak = 3x. Storage: 100M new URLs/month x 500 bytes = 50GB/month. This tells you reads are dominant (cache-friendly) and storage is modest.
Test Yourself
A social app has 10M DAU. Each user posts once per day and reads 50 posts. Estimate read QPS and write QPS at peak (3x average).
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