RAID Calculator: Usable Capacity & Fault Tolerance
Calculate usable storage, efficiency and drive-failure tolerance for RAID 0, 1, 5, 6, 10, 50, 60 and JBOD arrays. Enter your drive count and capacity to compare RAID levels before you buy drives or configure a server.
- Free, no sign-up
- 9 RAID levels
- Runs in your browser
- Shareable link
Array configuration
Results
RAID 5 · 8 × 4 TBFault tolerance: any 1 drive. Read speed is relative to one drive; write penalty is disk I/Os per random write.
Parity is distributed across all drives in practice; the tiles show the capacity it uses (1 drive).
RAID levels compared
Live values for 8 × 4 TB drives · RAID 50/60 with 2 groups| Level | Formula | Min. drives | Usable | Efficiency | Survives | Typical use | Select |
|---|
N = number of drives, S = capacity of one drive, G = span groups. Figures use decimal units as printed on drive labels and exclude hot spares, controller metadata and file-system overhead.
RAID calculator FAQ
How do I calculate usable RAID capacity?
Multiply the number of drives that hold data by the size of one drive. RAID 5 gives up one drive to parity, so 8 × 4 TB drives give (8 − 1) × 4 = 28 TB. RAID 6 gives up two drives, RAID 10 gives up half, and RAID 50 and 60 give up one or two drives per span group.
Should I choose RAID 5 or RAID 6?
RAID 6 for large drives or large arrays. A rebuild after a failure can take a day or more on multi-terabyte drives, and RAID 6 keeps protection during that rebuild where RAID 5 has none. RAID 5 suits smaller, read-heavy arrays where capacity matters more than a second parity drive.
Is RAID 10 better than RAID 5?
For write-heavy work such as databases and virtual machines, yes: writes cost two disk operations instead of four, and a rebuild only copies one mirror. The price is capacity: RAID 10 keeps half of the raw space, RAID 5 keeps all but one drive.
Why does my server show less space than the calculator?
Drives are labelled in decimal terabytes while operating systems report binary tebibytes, so 28 TB appears as about 25.5 TiB. The controller also reserves a little space for metadata, a hot spare holds no data, and the file system adds its own overhead.
Can I mix drive sizes in a RAID array?
You can, but every drive is treated as the size of the smallest one, so the extra capacity on larger drives is wasted. Mixing speeds or interfaces also drags the array down to the slowest member. Use matched drives where you can.
Is RAID a backup?
No. RAID protects against drive failure, not against deletion, ransomware, corruption, theft or fire, all of which are mirrored faithfully across the array. Keep a separate backup, ideally off site.