RAID Capacity Calculator (RAID 0, 1, 5, 6, 10)
How much usable storage do you actually get from a RAID array? It depends on the level. RAID 0 keeps 100% of the raw space but tolerates zero failures, RAID 5 loses one disk to parity, RAID 6 loses two, and RAID 1 / RAID 10 cut your space in half for mirroring. Enter your number of disks and the capacity per drive below to get the usable TB and the number of disk failures you can survive — for RAID 0, 1, 5, 6 and 10. Free, no signup, runs entirely in your browser.
Usable RAID capacity depends on the level: RAID 0 = N × disk size (no redundancy); RAID 1 = size of one disk; RAID 5 = (N − 1) × disk size (tolerates 1 failure); RAID 6 = (N − 2) × disk size (tolerates 2 failures); RAID 10 = (N ÷ 2) × disk size (mirror + stripe). Example: 4 × 4 TB in RAID 5 = (4 − 1) × 4 = 12 TB usable.
When to use this calculator
- Size a NAS or server array before buying drives
- Compare usable capacity across RAID 0, 1, 5, 6 and 10
- Plan parity overhead and storage efficiency for a budget
- Check how many disk failures a configuration can survive
- Quick reference for IT students and sysadmins
Worked Example: 4 × 4 TB in RAID 5
- RAID 5 formula: usable = (N − 1) × disk size
- N = 4 disks, disk size = 4 TB
- usable = (4 − 1) × 4 = 3 × 4 = 12 TB
- Parity overhead = 4 TB (one disk's worth), tolerates 1 failure
How it works
2 min readA RAID capacity calculator tells you how much usable storage you keep after redundancy overhead. The raw total is always number of disks × capacity per disk, but each RAID level reserves part of that for parity or mirroring.
Usable capacity formula by RAID level
| RAID level | Usable capacity | Disk failures tolerated | Min. disks |
|---|---|---|---|
| RAID 0 | N × size | 0 | 2 |
| RAID 1 | 1 × size | N − 1 | 2 |
| RAID 5 | (N − 1) × size | 1 | 3 |
| RAID 6 | (N − 2) × size | 2 | 4 |
| RAID 10 | (N ÷ 2) × size | 1 per mirror | 4 |
(N = number of disks, size = capacity of one disk. Assumes identical drives.)
Common configurations (usable capacity)
| Configuration | Raw total | RAID 0 | RAID 1 | RAID 5 | RAID 6 | RAID 10 |
|---|---|---|---|---|---|---|
| 2 × 4 TB | 8 TB | 8 TB | 4 TB | — | — | — |
| 4 × 4 TB | 16 TB | 16 TB | 4 TB | 12 TB | 8 TB | 8 TB |
| 4 × 8 TB | 32 TB | 32 TB | 8 TB | 24 TB | 16 TB | 16 TB |
| 6 × 4 TB | 24 TB | 24 TB | 4 TB | 20 TB | 16 TB | 12 TB |
| 8 × 8 TB | 64 TB | 64 TB | 8 TB | 56 TB | 48 TB | 32 TB |
How efficiency scales
Storage efficiency = usable ÷ raw. RAID 0 is 100% efficient but has no protection. RAID 1 and RAID 10 are always 50% efficient. RAID 5 and RAID 6 get more efficient as you add disks: RAID 5 with 4 disks is 75% efficient (3/4), but with 8 disks it reaches 87.5% (7/8). RAID 6 with 4 disks is 50% (2/4), rising to 75% (6/8) at 8 disks — which is why wide RAID 6 arrays are popular for large NAS builds.
Choosing a level
Important caveats
Real usable space is slightly lower than these figures: filesystem overhead, the decimal-vs-binary TB/TiB difference (a "4 TB" drive is ~3.64 TiB), and reserved blocks all reduce it. These formulas give the array-level usable capacity before the filesystem.
Frequently asked questions
How do I calculate RAID 5 usable capacity?
RAID 5 usable capacity = (N − 1) × capacity per disk, where N is the number of drives. One disk's worth of space is reserved for distributed parity. Example: 4 × 4 TB in RAID 5 gives (4 − 1) × 4 = 12 TB usable, and the array survives one disk failure.
How much usable space does RAID 6 give?
RAID 6 usable capacity = (N − 2) × capacity per disk, because two disks' worth of space is reserved for dual parity. With 8 × 8 TB drives you get (8 − 2) × 8 = 48 TB usable and can survive two simultaneous failures.
Why does RAID 1 only give 50% capacity?
RAID 1 mirrors the same data onto every drive, so usable capacity equals the size of a single disk regardless of how many drives you add. Two 4 TB drives give 4 TB usable, not 8 TB — the second copy is the redundancy.
What is the usable capacity of RAID 10?
RAID 10 usable capacity = (N ÷ 2) × capacity per disk, since data is mirrored and then striped. It requires an even number of at least 4 drives. Four 4 TB drives give (4 ÷ 2) × 4 = 8 TB usable.
Does RAID 0 lose any capacity?
No. RAID 0 uses 100% of the raw space (N × disk size) by striping data across all drives. The trade-off is zero redundancy: if any single drive fails, the entire array's data is lost.
How many disk failures can each RAID level survive?
RAID 0 tolerates zero failures. RAID 5 tolerates one. RAID 6 tolerates two. RAID 1 tolerates N − 1 (any but the last mirror copy). RAID 10 tolerates one failure per mirror pair, so it can survive multiple failures as long as no full pair is lost.
Why is my real usable space lower than the calculator shows?
Three reasons: filesystem overhead and reserved blocks, the binary-vs-decimal unit gap (a "4 TB" drive is about 3.64 TiB as reported by your OS), and metadata. These formulas give array-level capacity before the filesystem, so expect roughly 5–10% less after formatting.
Do the drives have to be the same size?
For predictable results, yes. If you mix sizes, most RAID levels treat every disk as if it were the size of the smallest one, so the extra space on larger drives is wasted. This calculator assumes identical drives.
Is RAID a substitute for backups?
No. RAID protects against drive hardware failure, not against accidental deletion, ransomware, fire, or controller failure. Always keep separate backups — RAID is for uptime and availability, not for backup.
Which RAID level is best for a NAS?
For most home and small-business NAS builds, RAID 5 (3–5 drives) or RAID 6 (6+ drives, or any array with large drives) gives the best balance of usable capacity and protection. RAID 10 is preferred when write performance and fast rebuilds matter more than capacity.