Estimation cheat sheet: numbers every engineer should know
The latency numbers, storage conversion rules, and throughput estimates you need for back-of-envelope calculations in system design interviews. Plus a worked example showing how these numbers combine to guide design decisions.
5 min read2026-04-04easyframeworkestimationreferenceinterview-prep
Memorize the latency tiers: memory ~100ns, SSD ~1ms, same-datacenter network ~500µs, cross-US ~40ms, cross-Atlantic ~80ms. Ratios (not exact numbers) are what matter for design decisions.
Storage: 2^10 = 1KB, 2^20 = 1MB, 2^30 = 1GB, 2^40 = 1TB. A day has ~86,400 ≈ 10^5 seconds. Round aggressively to powers of 10 — interview estimation is order-of-magnitude reasoning, not accounting.
A single PostgreSQL primary handles 1-20k writes/sec depending on query complexity and index overhead. A Redis instance handles 100-500k ops/sec. These numbers tell you when you need horizontal scaling.
For read/write rate estimation: start with DAU × average operations per day, divide by 86,400 to get per-second rate. Add a 2-3× peak multiplier for traffic spikes.
Let the numbers guide decisions: 600 TB/day upload → object storage choice. 2,300 uploads/sec → manageable with a small fleet. 11,500 reads/sec → CDN is required to avoid serving all from S3 directly.