RAID 5 Calculator
RAID 5 stripes data with single distributed parity — good space efficiency and tolerates one disk failure, with a well-known rebuild-time risk on large arrays.
The RAID 5 formula
Usable Capacity = (N − 1) × Disk Size
One disk's worth of capacity is reserved for parity, spread evenly across every disk rather than dedicated to a single one. Efficiency improves as you add disks — a 4-disk array is 75% efficient, a 10-disk array is 90% efficient.
Worked example
5 disks × 4 TB = 16 TB usable out of 20 TB raw, 80% storage efficiency, tolerates 1 disk failure. Read speed scales with disk count; writes are slower than RAID 0 due to the parity calculation on every write.
Why RAID 5 gets risky on large arrays
Rebuilding a failed disk means reading every remaining disk in full to reconstruct the missing data. On 8 TB+ drives that read can take the better part of a day, during which the array has zero redundancy — if a second disk fails or hits an unrecoverable read error during that window, RAID 5 loses data outright. This is why most admins now default to RAID 6 for arrays with more than 6-8 large disks, reserving RAID 5 for smaller arrays or SSD-based arrays with fast rebuilds.
What's the minimum number of disks for RAID 5?
3 disks. Below that, you're better off with RAID 1 (2 disks) — RAID 5's parity math doesn't provide any real advantage at that size.
Why does RAID 5 have a "write penalty"?
Every write requires reading the existing data and parity block, recalculating parity, then writing both back — this read-modify-write cycle (often called the RAID 5 write penalty) makes small random writes noticeably slower than sequential writes or than RAID 0/1.
Need a different RAID level? Use the full RAID Calculator covering RAID 0, 1, 5, 6, 10, 50, 60 and ZFS RAIDZ1/2/3, or read the RAID levels comparison guide.