RAID Capacity Calculator
Estimate usable storage, parity overhead, and fault tolerance for common RAID levels.
What this RAID space calculator tells you
A RAID setup is more than “number of drives × drive size.” Depending on the RAID level, some disk space is used for redundancy (parity or mirroring), and some may be reserved for hot spares. This RAID space calculator gives you a practical estimate of:
- Raw installed capacity
- Array raw capacity after hot spares
- Usable RAID capacity before filesystem overhead
- Estimated usable capacity after overhead
- Approximate fault tolerance by RAID type
The calculator assumes all active drives are the same size. In mixed-drive arrays, real systems usually limit each drive to the size of the smallest drive in the pool.
RAID formulas at a glance
| RAID Level | Minimum Drives | Usable Capacity Formula | Typical Fault Tolerance |
|---|---|---|---|
| RAID 0 | 2 | N × S |
0 drives (no redundancy) |
| RAID 1 | 2 | 1 × S (full mirror set) |
Up to N-1 if one copy remains |
| RAID 5 | 3 | (N - 1) × S |
1 drive |
| RAID 6 | 4 | (N - 2) × S |
2 drives |
| RAID 10 | 4 (even) | (N / 2) × S |
At least 1, up to N/2 depending on pair failures |
Where N is the number of active (non-spare) drives and S is the drive size.
Example: 4 × 8 TB in RAID 5
Step-by-step
- Raw installed =
4 × 8 TB = 32 TB - RAID 5 usable before filesystem overhead =
(4 - 1) × 8 TB = 24 TB - If you reserve 2% for metadata/filesystem overhead, estimated usable =
24 × 0.98 = 23.52 TB
This is why many users are surprised when they do not see the “full” advertised disk size after creating an array. Parity, mirroring, metadata, and binary/decimal unit differences all reduce visible space.
How to choose the right RAID level
RAID 0: performance first, no protection
RAID 0 offers maximum capacity and speed, but no redundancy. If any drive fails, the full array is lost. Best used only for temporary scratch data or workloads with strong external backups.
RAID 1: simple mirroring
RAID 1 is easy to understand and recover, but capacity efficiency is low in full mirrored sets. You trade usable space for resilience and read performance.
RAID 5: capacity-efficient protection
RAID 5 is common in home labs and small business NAS systems because it balances usable capacity and fault tolerance. It survives one drive failure, but rebuild stress can be significant on very large disks.
RAID 6: safer for larger arrays
RAID 6 sacrifices extra capacity for dual-parity protection. It is often a better default when arrays are large, disks are big, or rebuild windows are long.
RAID 10: performance and redundancy
RAID 10 combines striping and mirroring. Capacity efficiency is lower than RAID 5/6, but write performance and rebuild behavior are often better for transactional workloads.
Important real-world considerations
- Drive size mismatch: arrays generally align to the smallest active drive.
- Binary vs decimal units: operating systems may show TiB while drives are sold in TB.
- Controller overhead: metadata and formatting consume some space.
- Hot spares: improve resilience but reduce immediately usable capacity.
- RAID is not backup: accidental deletion, ransomware, and corruption can still destroy data.
Quick FAQ
Does RAID increase total capacity?
Some levels do (RAID 0), some reduce it substantially (RAID 1/10), and some use parity overhead (RAID 5/6). The calculator helps you estimate the tradeoff before buying drives.
Should I pick RAID 5 or RAID 6?
If data availability is critical and your disks are large, RAID 6 is typically safer. RAID 5 gives more usable space but only single-drive fault tolerance.
Can I ignore filesystem overhead?
You can set overhead to 0% for a simple estimate, but including 1–5% usually gives a more realistic result.
Bottom line
A good RAID plan starts with capacity math, but it should also include reliability goals, rebuild risk, performance needs, and a backup strategy. Use this RAID space calculator to size your array quickly, then validate against your NAS or RAID controller documentation for production decisions.