RAID 6 Calculator
RAID 6 adds a second independent parity calculation over RAID 5, tolerating two simultaneous disk failures — the safer default for large arrays on modern high-capacity drives.
The RAID 6 formula
Usable Capacity = (N − 2) × Disk Size
Two disks' worth of capacity go to double parity, spread across the array. Compared to RAID 5 on the same disk count, you lose one extra disk of raw capacity — in exchange for surviving two simultaneous failures instead of one.
Worked example
8 disks × 8 TB = 48 TB usable out of 64 TB raw, 75% storage efficiency, tolerates 2 disk failures. On the same 8 disks, RAID 5 would give 56 TB (87.5% efficiency) but only survive 1 failure.
Why RAID 6 has become the default over RAID 5
Rebuilding a large modern drive (8 TB+) after a failure can take the better part of a day, during which a RAID 5 array runs with zero redundancy. RAID 6 survives a second failure — or an unrecoverable read error — during that exact rebuild window, which is precisely the failure mode that causes real-world RAID 5 data loss. Most storage vendors now recommend RAID 6 by default for any array with more than 5-6 disks or drives 4 TB and larger. See the full RAID 6 usable capacity guide for more detail.
What's the minimum number of disks for RAID 6?
4 disks. Below that, the double-parity overhead (50% at 4 disks) offers no real advantage over RAID 10, which rebuilds faster.
How much slower is RAID 6 than RAID 5 for writes?
Somewhat slower — RAID 6 calculates two independent parity blocks per write instead of one, adding extra CPU/controller overhead. In practice the difference is usually modest on modern hardware RAID controllers with dedicated parity engines, more noticeable on software RAID.
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.