Storage Estimation Playbook
The formula
Storage = writes/day × bytes/write × 365 × years × replication factor
Then add indexes/metadata overhead (~10–30%) and don’t forget replication (×3 is the common default).
Steps
- Writes/day from QPS:
write_QPS × 86,400. - Bytes/write: use the “bytes per thing” anchors (text ~300 B, row ~1 KB, photo ~1 MB).
- Multiply by retention in days, then by replication.
Worked example — a photo service
10M photos/day, ~1.5 MB each (compressed + a thumbnail), kept 5 years, ×3 replication:
- Per day: 10⁷ × 1.5 MB = 15 TB/day.
- 5 years: 15 TB × 365 × 5 ≈ ~27 PB, ×3 replication ≈ ~80 PB.
- Gate: petabytes of immutable blobs ⇒ object storage (S3), not a database.
Common mistakes
- Forgetting replication (off by 3×) and indexes/overhead.
- Confusing storage (bytes) with bandwidth (bits) — see the Gbps vs GB/s trap.
Formulas are standard/public-domain engineering math. Approach and reference-table format adapted from the System Design Primer (CC BY 4.0), Jeff Dean’s latency numbers, the DesignGurus capacity-estimation guide, and Little’s Law.
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