RAID Capacity Calculator
Estimate usable storage for common RAID levels (0, 1, 5, 6, 10). This calculator assumes all drives are the same size.
What this RAID drive calculator helps you decide
If you are planning a NAS, home server, lab cluster, or small office backup appliance, one of the first questions is simple: how much usable capacity will I really get? A RAID drive calculator answers that in seconds.
When people buy drives, they often multiply drive count by drive size and assume that is the final total. In real life, RAID redundancy, parity, and filesystem overhead all reduce usable space. This page gives you a practical estimate before you buy hardware.
RAID capacity formulas used in this calculator
The logic below assumes identical drive sizes and standard RAID behavior:
RAID 0 (striping)
- Minimum drives: 1
- Usable capacity:
N × drive_size - Fault tolerance: none
RAID 0 maximizes speed and space but has no redundancy. One failed disk means total array loss.
RAID 1 (mirroring)
- Minimum drives: 2
- Usable capacity:
1 × drive_size(for a fully mirrored set) - Fault tolerance: up to
N - 1drives
Great for simplicity and availability, but capacity efficiency is low when drive count rises.
RAID 5 (single parity)
- Minimum drives: 3
- Usable capacity:
(N - 1) × drive_size - Fault tolerance: 1 drive
RAID 5 is capacity-efficient but rebuild stress can be significant on large drives.
RAID 6 (dual parity)
- Minimum drives: 4
- Usable capacity:
(N - 2) × drive_size - Fault tolerance: 2 drives
RAID 6 offers stronger protection during long rebuild windows and is common in larger arrays.
RAID 10 (1+0)
- Minimum drives: 4 (even count)
- Usable capacity:
(N / 2) × drive_size - Fault tolerance: at least 1 drive, potentially more if failures are in different mirror pairs
RAID 10 gives excellent performance and resilient rebuild behavior, with 50% raw-capacity efficiency.
How to interpret your result
The calculator reports:
- Raw capacity: total space of active data drives before RAID math.
- Usable RAID capacity: expected space after parity or mirroring overhead.
- Estimated usable after filesystem overhead: a practical value closer to what you can store.
- Capacity efficiency: percentage of raw capacity available to data.
- Fault tolerance: how many drives can fail without total data loss.
Real-world planning tips
1) Treat RAID as availability, not backup
RAID protects against drive failure, not accidental deletion, ransomware, corruption, fire, or theft. You still need separate backups (preferably offsite or immutable).
2) Use identical drives when possible
Most arrays are limited by the smallest disk. Mixing drive sizes can reduce total usable storage and complicate expansion.
3) Expect longer rebuilds with larger disks
As drives get bigger, rebuild windows grow. During rebuilds, arrays face higher risk and lower performance. This is why many admins pick RAID 6 instead of RAID 5 for large-capacity systems.
4) Keep one hot spare for critical systems
A hot spare can automatically replace a failed drive and start rebuild immediately, reducing time spent in a degraded state.
Example: quick sizing before purchase
Suppose you plan a 6-drive NAS with 12 TB disks in RAID 6 and no hot spare:
- Raw capacity: 72 TB
- Usable RAID capacity: 48 TB
- After 2% overhead: about 47.04 TB
- Fault tolerance: any 2 drives
That is a much more realistic planning number than the raw 72 TB on the box.
FAQ
Is TB the same as TiB?
No. Drive vendors usually advertise decimal TB. Operating systems often report binary TiB. This calculator displays both to reduce surprises.
Can I use this for ZFS, mdadm, Synology, or hardware RAID?
Yes as a quick estimate. Exact values vary by implementation, metadata layout, record size, snapshots, and reserved space settings.
What if I have mixed drive capacities?
Use the smallest drive size for a conservative estimate, or plan separate vdevs/arrays by size class.
Bottom line
A RAID drive calculator gives you an immediate, practical capacity estimate so you can choose the right RAID level, drive count, and budget before buying. Use it as your first pass, then confirm with your platform-specific storage manager before deployment.