HLD overview
Tool
Capacity estimation calculator
Change the inputs and read the numbers out loud in your interview. Every figure is a back-of-the-envelope estimate, not a benchmark.
Writes / day50.00M
Write QPS (avg)578.70
Read QPS (avg)57.87K
Peak write QPS1.74K
Peak read QPS173.61K
New storage / day95.37 GB
Storage after 365d (×3)101.98 TB
Write bandwidth1.13 MB/s
Read bandwidth113.03 MB/s
Cache working set19.07 GB
Numbers worth memorising
- 1 million writes/day ≈ 12 writes/second average.
- Peak traffic is usually 2–5× the daily average.
- A single commodity SQL node handles roughly 1–5K simple QPS before you shard.
- Redis serves 100K+ ops/second per node with sub-millisecond latency.
- Assume 80/20: 20% of the data serves 80% of the reads — that's your cache size.