Interactive Engineering Tool
Back-of-the-Envelope Capacity Math Calculator
Derive Peak QPS, Bandwidth, Daily Storage, 5-Year Storage Growth, and 80/20 RAM Caching budgets in seconds for your system design interview.
Interactive Calculator
Configure System Design Parameters
Adjust inputs to calculate architectural scale numbers in real time.
| Calculated Metric | Derived Capacity Value | Interview Talk Track Context |
|---|---|---|
| Average Throughput (QPS) | 11,574 QPS | Total requests per day ÷ 86,400 seconds |
| Peak Throughput (2.5× Surge) | 28,935 Peak QPS | Size auto-scaling web compute and ingress load balancers |
| Read QPS vs Write QPS | 9,259 Read / 2,315 Write | Justifies read replicas vs partitioned write shards |
| Peak Ingress/Egress Bandwidth | 57 MB/s | Peak QPS × payload size; sizes network links and CDN egress |
| Daily Storage Growth | 381 GB / day | Write requests × payload size accumulated over 24 hours |
| 5-Year Database Storage | 679 Terabytes (TB) | Determines distributed sharding key and cold storage archival |
| RAM Cache Budget (80/20 Rule) | 305 GB RAM | Cache the top 20% most active read items in Redis/Memcached |
| Storage IOPS at Peak | 10,417 IOPS | Peak writes plus the 19.999999999999996% of peak reads that miss the cache |
Capacity math in live practice
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