Estimates help identify orders of magnitude and likely bottlenecks. State assumptions and round numbers instead of pretending estimates are exact.

Traffic formulas

• Average requests per second = daily requests / 86,400.
• Peak RPS = average RPS × peak-to-average multiplier.
• Write RPS = total RPS × write fraction.
• Read RPS = total RPS × read fraction.
• Approximate concurrent requests = RPS × average request duration in seconds.

Storage formulas

• Raw storage = records per day × average record size × retention days.
• Replicated storage = raw storage × replication factor.
• Total storage should also account for indexes, metadata, logs, and growth overhead.
• Image/video storage = uploads per day × average object size × retention days.

Bandwidth formulas

• Average bandwidth in bytes/second = RPS × average payload size in bytes.
• Approximate bits/second = bytes/second × 8.
• Outbound bandwidth may dominate when responses contain large media objects.

Example: 10 million daily active users Assume each user generates 20 requests per day.

• Daily requests = 10,000,000 × 20 = 200,000,000.
• Average RPS = 200,000,000 / 86,400 ≈ 2,315.
• If peak traffic is 4× average, peak RPS ≈ 9,260.
• If 80% are reads, peak read RPS ≈ 7,408 and write RPS ≈ 1,852.

Example: storage growth Assume 5 million new records per day, each averaging 1 KB, retained for 365 days.

• Raw daily growth ≈ 5 GB using decimal units.
• Annual raw data ≈ 1.825 TB.
• With three copies, replicated data alone is approximately 5.475 TB, excluding indexes and overhead.

Useful relationship: Little's Law Average in-flight work = throughput × average time in the system. At 2,000 requests/second and 0.2 seconds average latency, expect approximately 400 requests in flight.

Interview checklist: specify units, distinguish average from peak, include growth and retention, estimate payload sizes, and identify which assumptions most affect the result.