RAID (redundant array of independent disks) combines several drives into one volume to gain capacity, speed, protection against drive failure, or a mix of the three. Each level trades usable space for redundancy differently, so the same four drives can give you anywhere from one drive’s worth of space to all four. This calculator shows the usable capacity of RAID 0, 1, 5, 6 and 10 for your drives, how much goes to parity or mirrors, how many failures the array survives and how the levels compare.
How to use the RAID calculator
- Choose the RAID level.
- Enter the number of drives and the capacity of each drive (TB, GB or TiB). With mixed sizes, enter the smallest.
- Optionally enter hot spares: idle drives that join the array automatically after a failure. They count toward raw capacity but add no usable space.
- Read the usable capacity in decimal TB and in TiB as an operating system will report it, then the redundancy overhead, fault tolerance and relative performance. The table compares every level for the same drives.
RAID capacity formulas
With n array drives of size S:
| Level | Usable capacity | Minimum drives | Survives |
|---|---|---|---|
| RAID 0 (striping) | n × S | 2 | No failures |
| RAID 1 (mirroring) | 1 × S | 2 | n − 1 failures |
| RAID 5 (single parity) | (n − 1) × S | 3 | 1 failure |
| RAID 6 (double parity) | (n − 2) × S | 4 | Any 2 failures |
| RAID 10 (striped mirrors) | n/2 × S | 4, even | 1 guaranteed, up to n/2 |
Worked example
A small office NAS has four 8 TB drives in RAID 5.
Usable: (4 − 1) × 8 TB = 24 TB; raw capacity is 32 TB, so 8 TB holds parity.
Efficiency: 3 ÷ 4 = 75%. The array survives one drive failure.
What the operating system shows: 24 × 10¹² ÷ 1,099,511,627,776 = 21.83 TiB.
Same drives in RAID 6 or RAID 10: 16 TB usable (50%), but RAID 6 survives any two failures.
With six 4 TB drives in RAID 5 plus one hot spare, the array uses five drives: (5 − 1) × 4 = 16 TB usable from 24 TB raw.
Performance at a glance
| Level | Sequential read | Sequential write | Random-write penalty |
|---|---|---|---|
| RAID 0 | up to n× | up to n× | 1 I/O |
| RAID 1 | up to n× | 1× | n I/Os |
| RAID 5 | up to (n − 1)× | up to (n − 1)× | 4 I/Os |
| RAID 6 | up to (n − 2)× | up to (n − 2)× | 6 I/Os |
| RAID 10 | up to n× | up to n/2× | 2 I/Os |
The write penalty is the number of disk operations a small random write costs: RAID 5 must read old data and parity, then write new data and parity (4 I/Os); RAID 6 updates two parity blocks (6). That is why RAID 10 is preferred for write-heavy databases. Real throughput also depends on the controller, cache, file system and network.
Choosing a level
- RAID 1 — two drives, simple, ideal for a boot volume or a small home NAS.
- RAID 5 — good capacity with single-drive protection, but rebuilding a large array reads every remaining drive in full, and a second failure or read error during that rebuild loses data. Many administrators avoid RAID 5 with drives above a few terabytes.
- RAID 6 — the safer choice for large, mostly-read arrays such as media and backup storage.
- RAID 10 — best random-write performance and fast rebuilds, at 50% efficiency.
- RAID 0 — speed only; use it for scratch data you can lose.
To convert between TB, TiB and other units, use the data storage converter. For how long it takes to copy data onto a new array over the network, try the bandwidth calculator.
Frequently asked questions
How much usable space does RAID 5 give?
The capacity of all drives minus one: (n − 1) × drive size. Four 8 TB drives in RAID 5 give 3 × 8 = 24 TB, which an operating system shows as about 21.8 TiB.
RAID 6 or RAID 10?
RAID 6 survives any two drive failures and keeps more capacity as the array grows, which suits large drives and archival storage. RAID 10 survives one guaranteed failure (more if they hit different mirror pairs), rebuilds faster and handles random writes better, which suits databases and virtual machines.
Why does my 8 TB drive show only 7.28 TB?
Drive makers use decimal terabytes (10¹² bytes) while most operating systems divide by 2⁴⁰ and label the result TB, though it is really TiB. 8 × 10¹² ÷ 1,099,511,627,776 = 7.28 TiB. No space is missing.
Can I mix drive sizes in an array?
Traditional RAID uses the smallest drive's capacity on every member, so the extra space on larger drives is wasted. Some NAS systems offer flexible schemes that use mixed sizes better, but for standard RAID enter the smallest drive in the calculator.
Is RAID a backup?
No. RAID keeps a system running through a drive failure, but it copies deletions, ransomware encryption and corruption to every drive instantly, and it does not protect against theft, fire or controller faults. Keep separate backups, ideally including an off-site copy.