raid 6 space calculator

How RAID 6 capacity works

RAID 6 uses block-level striping with dual distributed parity. In practical terms, the array can tolerate the failure of any two active drives without losing data. The tradeoff is space: the equivalent capacity of two drives is consumed by parity across the array.

The classic usable-capacity formula is:

Usable = (Number of Active Drives - 2) × Smallest Drive Size

If all drives are identical and no hot spares are reserved, that formula is usually accurate enough for planning. Real systems can report slightly less due to binary/decimal unit differences and filesystem overhead.

Quick example

8 drives × 12 TB each (no hot spare)

• Raw installed: 96 TB
• Parity cost in RAID 6: 24 TB (2 drives)
• Usable before formatting: 72 TB

If you reserve one hot spare, only 7 drives are active in the RAID set. Then usable becomes (7 - 2) × 12 = 60 TB before overhead.

Why your operating system shows less space than expected

There are three common reasons reported capacity looks smaller:

• Decimal vs binary units: vendors market in TB (10¹² bytes), while many systems display TiB (2⁴⁰ bytes).
• RAID metadata: controllers and software RAID reserve a small amount of space.
• Filesystem overhead: structures such as journaling, checksums, and snapshots consume additional capacity.

This calculator includes an overhead field so your estimate better reflects real-world usable space.

RAID 6 planning tips

1) Keep drive sizes matched

Mixed drive sizes usually reduce effective capacity to the smallest common size across members. For predictable outcomes, use identical models where possible.

2) Consider rebuild time

Large arrays with large disks can take many hours (or longer) to rebuild after a failure. RAID 6 helps by tolerating a second failure during rebuild, but you should still monitor health and replace failed drives quickly.

3) Use hot spares when uptime matters

A hot spare does not increase normal usable capacity, but it can reduce the time your array stays in a degraded state by starting rebuild automatically.

4) RAID is not backup

RAID improves availability. It does not protect against accidental deletion, ransomware, silent corruption outside the array’s protection scope, or site-wide disasters. Keep offline/offsite backups.

RAID 6 vs RAID 5 vs RAID 10 (capacity perspective)

• RAID 5: loses capacity of one drive; better capacity efficiency than RAID 6, but less fault tolerance.
• RAID 6: loses capacity of two drives; better resilience for larger arrays.
• RAID 10: approximately 50% usable capacity; strong performance and rebuild behavior, but highest capacity cost.

Final takeaway

For RAID 6, start with the two-drive parity rule, then subtract any reserved hot spares and your expected overhead. The calculator above gives you a fast, practical estimate so you can size storage confidently before purchasing hardware.