raid calculator

What this RAID calculator does

This RAID calculator helps you quickly answer the practical question every storage planner asks: How much space will I really get? Enter your RAID level, drive count, and drive size, and the tool estimates raw capacity, usable capacity, redundancy overhead, and drive-failure tolerance.

It is designed for fast planning in homelabs, NAS builds, and small business servers. It assumes all drives in the array are equal size and that your array uses standard RAID behavior for parity or mirroring.

Quick RAID level reference

RAID 0

RAID 0 stripes data across drives for performance and full capacity usage. The tradeoff is severe: if one drive fails, all array data is lost.

RAID 1

RAID 1 mirrors data. In a simple mirror, usable capacity is the size of one drive, while the other drive(s) hold copies. It provides excellent read performance and strong redundancy for small arrays.

RAID 5

RAID 5 uses distributed parity and can survive one failed drive. Capacity is typically (N - 1) × drive size, where N is active drives.

RAID 6

RAID 6 adds dual parity and can survive two failed drives. Capacity is usually (N - 2) × drive size. This is popular for large-capacity arrays where rebuild risk is higher.

RAID 10

RAID 10 combines striping and mirroring. It needs an even number of drives (minimum 4), offers strong performance, and usually provides half raw capacity as usable capacity.

How to use the calculator correctly

• Use the actual count of installed drives for total physical drives.
• Set hot spares if you dedicate standby drives not holding active data.
• Use the same capacity unit as your hardware vendor (normally TB).
• Treat results as estimates; controller behavior and filesystem overhead can reduce usable space further.

Important planning notes

1) RAID is not backup

RAID protects availability, not historical recovery. You still need backups for accidental deletion, malware, corruption, and site-level disasters.

2) Large drives increase rebuild exposure

As drive size grows, rebuild windows can become very long. During rebuild, stress on remaining drives rises. This is one reason many teams choose RAID 6 over RAID 5 at larger capacities.

3) Mixed drive sizes are constrained by the smallest drive

Most RAID implementations size every disk to the smallest member. If you mix capacities, usable space can be less than expected.

Example scenarios

Example A: 6 × 8 TB in RAID 5, no hot spare

• Raw capacity: 48 TB
• Usable estimate: 40 TB
• Fault tolerance: 1 failed drive

Example B: 8 × 12 TB in RAID 6 with 1 hot spare

• Active drives: 7
• Usable estimate: (7 - 2) × 12 = 60 TB
• Fault tolerance: 2 failed drives (before considering rebuild state)

Final takeaway

A RAID calculator is best used early in design: before buying drives, before selecting a chassis, and before setting growth targets. Small planning decisions (RAID level, spare policy, drive count) have large effects on both usable storage and operational risk. Use this tool as a first pass, then validate against your NAS or RAID controller documentation for production deployment.