Back-of-Envelope Calculator

Every design starts with estimation. Plug in your assumptions and get the numbers that shape the architecture: QPS tells you how many servers, storage tells you which database, bandwidth tells you whether you need a CDN.

Daily active userse.g. 10,000,000Writes per user per dayposts, uploads, messages…Read : write ratio10 means 10 reads per writeAverage object size (bytes)e.g. 1 KB post = 1000Peak traffic multiplierusually 2-5x averageReplication factorcopies of each object, usually 3Retention (years)how long data is kept

Write QPS (avg)

20.00M writes/day ÷ 86,400s

231

Read QPS (avg)

200.00M reads/day ÷ 86,400s

2.3K

Peak total QPS

(read + write QPS) × 2

5.1K

New storage / day

writes/day × object size (single copy)

20.00 GB

Total storage

5y retention × 3x replication

109.50 TB

Write bandwidth

ingress at average load

231.48 KB/s

Read bandwidth

egress at average load

2.31 MB/s

Cache size (80/20 rule)

hot 20% of a day's read volume

40.00 GB

Interview tip: round aggressively (86,400 ≈ 100K seconds/day) and state your assumptions out loud. The interviewer cares about your reasoning, not decimal places.