Back-of-envelope estimator
Interviewers want to see the arithmetic, not the answer. Use this to sanity-check numbers after you have done them by hand.
Numbers worth memorising
- 1 day ≈ 86,400 s ≈ 10⁵ s. So 1M/day ≈ 12/s, 100M/day ≈ 1,200/s.
- Memory read ~100 ns, SSD read ~100 µs, same-DC round trip ~0.5 ms, cross-continent ~150 ms.
- A single Postgres node comfortably serves thousands of simple QPS; Redis ~100K ops/s per node.
- 1 char = 1 B, UUID = 16 B, a timestamp = 8 B, a typical tweet ~300 B, a 1080p minute ~50 MB.
| Average read QPS | 2.31 K |
|---|---|
| Average write QPS | 231 |
| Peak read / write QPS | 6.94 K / 694 |
| New records per day | 20.00 M |
| Storage per day | 9.31 GB |
| Storage over 5 years | 16.60 TB |
| Read / write bandwidth (avg) | 1.10 MB/s / 113.03 KB/s |
| Cache for one day of hot data | 1.86 GB |