Essential Conversion Factors and Mental Models for Quick Calculations

Time Conversions
Seconds per day: 86,400 (round to 100,000 for easy math). Seconds per month: ~2.5 million. Seconds per year: ~30 million.
These let you convert between "per day" and "per second" instantly. 1 million requests/day = 1M / 100k = 10 requests/sec. 100 requests/sec = 100 × 100k = 10 million/day.
Data Size Conversions
1 KB = 1,000 bytes. 1 MB = 1,000 KB. 1 GB = 1,000 MB. 1 TB = 1,000 GB. 1 PB = 1,000 TB.
Common sizes: Character: 1 byte. Integer: 4 bytes. Timestamp: 8 bytes. UUID: 16 bytes. Average JSON object: 1KB. Small image: 100KB. Large image: 1MB. Video minute: ~100MB.
Bandwidth and Storage Speed
Network speeds are in bits, storage in bytes. 1 Gbps = 125 MB/sec (divide by 8). A gigabit network transfers 125MB every second, 450GB per hour.
SSD sequential write: ~500 MB/sec. SSD random operations: ~50,000/sec. Traditional hard drive sequential: ~100 MB/sec. Hard drive random operations: ~100/sec. This 500x difference in random operations is why SSDs transformed database performance.
Powers of 2 Reference
2^10 = 1,024 ≈ 1 thousand. 2^20 ≈ 1 million. 2^30 ≈ 1 billion. 2^40 ≈ 1 trillion.
This maps to data sizes: 2^10 bytes = 1KB, 2^20 = 1MB, 2^30 = 1GB, 2^40 = 1TB. When you see "32-bit address space," you know it addresses 2^32 = 4GB of memory.
Request Size Estimation
HTTP header: ~500 bytes. Simple API response: 1KB. List of 100 items: 100KB.
Bandwidth math: 1000 requests/sec × 1KB = 1 MB/sec = 8 Mbps. One server handles this easily. 100,000 requests/sec × 100KB = 10 GB/sec = 80 Gbps. This requires multiple servers and edge caching (servers placed geographically close to users).
Best Practice: Memorize five numbers: 100k seconds/day, 1 Gbps = 125 MB/sec, 2^10 ≈ 1k, 2^20 ≈ 1M, 2^30 ≈ 1B. These unlock most quick calculations.

💡 Key Takeaways

✓100k seconds per day simplifies daily to per-second conversion; 1M requests/day = 10 RPS, 100 RPS = 10M/day

✓Network is bits, storage is bytes; 1 Gbps = 125 MB/sec; a gigabit link moves 450GB per hour

✓Powers of 2: 2^10 ≈ 1k, 2^20 ≈ 1M, 2^30 ≈ 1B; maps directly to KB, MB, GB

✓Bandwidth math: 1000 RPS × 1KB = 8 Mbps (trivial), 100k RPS × 100KB = 80 Gbps (needs CDN)

📌 Interview Tips

1Convert fluently between daily and per-second: when told '50 million requests per day,' immediately respond '500 RPS average, probably 2000 RPS peak'

2Estimate bandwidth needs out loud: 'Each response is 10KB, at 1000 RPS that is 10MB/sec or 80 Mbps, well within a single server'

3Use powers of 2 for memory calculations: 'We need to store 1 million users at 1KB each, that is 2^20 × 2^10 = 2^30 = 1GB, fits in RAM'

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