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Back-of-envelope calculations for system design interviews. Pick a scenario, adjust parameters, get instant estimates.
Demonstrate capacity planning skills by estimating QPS, storage, and bandwidth before diving into architecture. Interviewers expect this as the first step.
Estimate infrastructure needs before launching a feature. Helps teams plan server provisioning, storage budgets, and CDN capacity.
Determine if you need horizontal scaling, caching layers, CDNs, or database sharding based on traffic estimates.
Convert storage/bandwidth estimates to cloud costs. 1 TB on S3 ~ $23/month, 1 TB egress ~ $90/month. Build business cases with data.
Explore our system design posts: Sharding · Replication · Cascade Failures · How Discord Stores Messages
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.