Raw capacity--
Usable capacity--
Failure tolerance--
RAID Storage Calculator for Usable Capacity
Estimate usable capacity and fault tolerance before building a NAS, small server, or storage array. RAID can improve availability, but it is not a backup.
Updated May 2026No signup requiredBuilt for mobile
Formula or method
Raw capacity = active drives x drive size.
Usable capacity depends on RAID level and redundancy overhead.
Worked examples
RAID 5 NAS
Four 8 TB drives in RAID 5 provide about 24 TB usable before filesystem formatting.
RAID 10 server
Six 4 TB drives in RAID 10 provide about 12 TB usable with mirrored pairs.
Practical use cases
- NAS planning
- server storage
- home lab storage
- media archive planning
- backup target sizing
Common mistakes
- Thinking RAID is a backup
- forgetting hot spares
- mixing drive sizes
- ignoring rebuild risk
FAQ
RAID Storage Calculator questions
How accurate is this calculator?
It is a planning estimate based on the values you enter. Real-world conditions can change the result.
Why do results vary?
Overhead, rounding, equipment limits, supplier units, network conditions, and user behavior can all affect the final number.
Should I round up?
Round conservatively when running short would interrupt a project, backup, stream, trip, or outage plan.
What should I do next?
Use the result card and checklist, then compare related calculators or guides before making a final decision.
Does this replace official documentation?
No. Use manufacturer documentation, platform guidance, or professional advice for critical decisions.