RAID Usable Capacity Calculator
A RAID array trades some raw capacity for redundancy, and how much you lose depends on the RAID level. Enter the number of drives, the capacity of each drive, and the RAID level. This calculator returns the usable capacity, the raw capacity, the capacity given up to redundancy, the storage efficiency as a percentage, and how many drives can fail without data loss. Use it to size an array before you buy disks.
RAID capacity formulas
RAID 0: usable = N * size
RAID 1: usable = size (mirror)
RAID 5: usable = (N - 1) * size
RAID 6: usable = (N - 2) * size
RAID 10: usable = N * size / 2 (N even)
N is the number of drives and size is the per-drive capacity. Efficiency is usable divided by raw capacity. RAID 5 needs at least 3 drives; RAID 6 needs at least 4; RAID 10 needs an even count of at least 4.
Example: four 4 TB drives (16 TB raw)
- RAID 0: 16 TB usable, no fault tolerance.
- RAID 5: 12 TB usable, survives 1 drive failure.
- RAID 6: 8 TB usable, survives 2 drive failures.
- RAID 10: 8 TB usable, survives 1 drive per mirror.
- RAID 1 (2 drives): one drive's capacity usable.
RAID capacity: frequently asked questions
How much usable space does each RAID level give?
RAID 0 uses all drives: capacity is drives times drive size. RAID 1 mirrors, so usable space equals one drive's size. RAID 5 loses one drive to parity: (drives minus 1) times drive size. RAID 6 loses two drives: (drives minus 2) times drive size. RAID 10 stripes mirrored pairs: drives times drive size divided by 2.
How many drives can fail without losing data?
RAID 0 tolerates no failures. RAID 5 survives one drive failure. RAID 6 survives two. RAID 1 and RAID 10 survive at least one failure, and RAID 10 can survive more as long as no full mirror pair is lost. This calculator reports the fault tolerance for the level you choose.
Why is RAID not a backup?
RAID protects against drive hardware failure, not against deletion, ransomware, corruption, or site loss. A file deleted on a RAID array is deleted on every disk at once. Always keep separate backups in addition to RAID redundancy.
Do all drives need to be the same size?
For best efficiency, yes. Most RAID levels treat every drive as if it were the size of the smallest member, so a larger drive wastes the extra space. This calculator assumes all drives are the size you enter.
Sources and definitions
- RAID capacity follows the standard parity and mirroring definitions for each level (RAID 0, 1, 5, 6, 10). These are standard storage definitions.
- National Institute of Standards and Technology: SI prefixes (tera = 10 to the 12th).
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.