Simplified Back-of-the-Envelope Calculation Cheat Sheet

Category Metric Value Notes
Time 1 second 1,000 milliseconds (ms) Useful for latency calculations.
1 day ~100,000 seconds Rounded up for easier estimation.
Data Size 1 kilobyte (KB) 103 bytes ~1,000 bytes (thousand).
1 megabyte (MB) 106 bytes ~1,000 KB (million).
1 gigabyte (GB) 109 bytes ~1,000 MB (billion).
1 terabyte (TB) 1012 bytes ~1,000 GB (trillion).
Network Bandwidth of 1 Gbps 125 MB/s 1 Gbps = 1,000 Mbps = 125 MB/s (divide by 8 to convert bits to bytes).
Round-trip time (RTT) ~100 ms (within a region) Assumes low latency within a data center or region.
Storage SSD latency ~0.1 ms (100 μs) Fast read/write times for SSDs.
HDD latency ~10 ms Slower than SSDs but cheaper for bulk storage.
Throughput Requests per second (RPS) ~1,000 RPS per server Depends on server capacity and workload.
Queries per second (QPS) ~10,000 QPS per database Depends on database type and optimization.
Memory RAM access time ~100 ns Much faster than disk access.
Cache access time (L1) ~1 ns Extremely fast access for frequently used data.
Users Daily Active Users (DAU) ~10% of total users Assumes 10% of users are active daily.
Monthly Active Users (MAU) ~30% of total users Assumes 30% of users are active monthly.
Traffic Reads vs. Writes ~90% reads, 10% writes Common for read-heavy systems (e.g., social media).
Peak traffic multiplier ~2x to 10x average traffic Plan for peak traffic spikes (e.g., Black Friday).
Miscellaneous UUID size 128 bits (16 bytes) Unique identifier size.
Compression ratio ~2x to 10x Depends on data type (e.g., text compresses better than images).

How to Use This Table

  1. Estimate Traffic: Use DAU/MAU and peak traffic multipliers to estimate requests per second.
  2. Calculate Bandwidth: Convert between bits and bytes to estimate network throughput.
  3. Compare Latencies: Use SSD/HDD/RAM latencies to decide storage and caching strategies.
  4. Size Data: Use data size conversions to estimate storage requirements.
  5. Plan for Scale: Use RPS/QPS estimates to determine the number of servers or databases needed.

Example Calculation