RAID Calculator
Choose a RAID level and enter your drive count to estimate storage available before formatting. Hot spares are excluded from the active array.
Equal drive sizes; capacity before formatting. RAID 1 uses a full mirror across all active disks.
Nominal usable capacity
24 TB
- Binary capacity
- 21.827873 TiB
- Installed raw capacity
- 32 TB
- Parity / mirror capacity
- 8 TB
- Hot spare capacity
- 0 TB
- Capacity efficiency
- 75%
- Drive failure tolerance
- Any one drive failure
What the capacity estimate includes
All disks in this model have the entered capacity. RAID 0 uses every active disk for data. RAID 5 reserves the equivalent of one disk for parity; RAID 6 reserves two. RAID 10 divides an even drive count into mirrored pairs, leaving half the active capacity usable.
RAID 1 here means a single full mirror across every active disk, so usable space equals one drive. A hot spare contributes no data capacity until it replaces a failed member. Filesystem metadata and controller reservations reduce the space you can actually use; neither is estimated here.
Capacity is only part of the decision
Four 8 TB disks in RAID 5 provide a nominal 24 TB, approximately 21.83 TiB. That difference comes from decimal versus binary units, not missing disks. Adding a fifth 8 TB disk as a spare leaves usable capacity unchanged.
Two failed disks in the same RAID 10 pair can destroy the array, even when the other pairs remain intact. Redundancy also cannot recover an accidentally deleted file by itself. Keep separate backups. For mixed drive sizes, vendor-specific layouts such as SHR can behave differently; this calculator does not model them or predict rebuild performance.