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Back-of-envelope calculations for system design interviews. Pick a scenario, adjust parameters, get instant estimates.
See these formulas in action with our worked example, or learn the fundamentals in our Distributed Systems guide.
There are 86,400 seconds in a day (24 × 60 × 60). Dividing daily operations by 86,400 gives average operations per second, assuming uniform distribution.
The Pareto principle: 20% of data accounts for 80% of requests. Caching the top 20% of daily data in memory can serve most read traffic without hitting the database.
Use 2x for steady workloads, 3x for social media with viral spikes, 5-10x for event-driven systems (ticket sales, flash sales, live sports).
In interviews, order of magnitude is sufficient. The difference between 10K and 100K QPS matters; 10K vs 12K does not. Focus on reasoning, not exact numbers.
Web servers: 5K-20K QPS. Database servers: 1K-10K QPS. Cache servers (Redis): 50K-100K QPS. API gateways: 10K-50K QPS. It depends on payload size and processing complexity.