RAID 6 Capacity Calculator
Calculate usable capacity of a RAID 6 array accounting for dual-disk parity.
About RAID 6 Capacity Calculator
RAID 6 extends RAID 5 by adding a second independent parity block, reserving the equivalent of two disks for parity data distributed across all members. This allows any two disks to fail simultaneously without data loss, making it safer for large arrays where the probability of a second failure during rebuild is non-trivial. The minimum disk count is 4.
Formulas
RAID 6 usable capacity
usable = (n − 2) × disk_size
- n
- number of disks
- S
- capacity per disk
Storage efficiency
efficiency = (n − 2) / n × 100%
- n
- number of disks
Worked example
With the default values below, RAID 6 Capacity Calculator returns Usable Capacity of 8,000.00. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Number of Disksminimum 4
- 6
- Size per Diskminimum 0.001 GB
- 2,000 GB
Results
- Usable Capacity
- 8,000.00 GB
- Raw Capacity
- 12,000.00 GB
- Parity Overhead
- 4,000.00 GB
- Storage Efficiency
- 66.67%
- Fault Tolerance
- Tolerates 2 simultaneous disk failures
Step by step
- 1Disk count (n)= 6
- 2Disk size= 2000 GB
- 3RAID 6 usable capacity= 8000 GB
- 4Dual-parity overhead= 4000 GB
- 5Storage efficiency= 66.67%
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
When should I choose RAID 6 over RAID 5?
Use RAID 6 for arrays with large-capacity drives (4TB+) where rebuild times are long and a second failure during rebuild would be catastrophic, or for arrays with more than 8 disks where statistical likelihood of correlated failure increases.
What is the write penalty for RAID 6?
RAID 6 has a higher write penalty than RAID 5 because each write operation must update two parity blocks instead of one — typically requiring 6 I/O operations per logical write vs 4 for RAID 5.