RAID Capacity Calculator

Frequently Asked Questions

What RAID level should I choose for a database?

RAID 10 is the preferred choice. It provides fast reads and writes with no parity penalty and tolerates one failure per mirrored pair. RAID 5/6 suffer a write penalty due to parity updates.

Why does RAID 6 require at least 4 drives?

RAID 6 uses double distributed parity, requiring two drives worth of parity data. With fewer than 4 drives there would be no usable capacity after allocating two parity blocks.

What is the TB vs TiB difference in practice?

Drive makers define 1 TB = 10^12 bytes (decimal). OSes define 1 TiB = 2^40 bytes (binary). The ratio is ~0.909, so an 8 TB RAID shows as ~7.27 TiB in the OS.

Can I add drives to an existing RAID array to expand capacity?

Some controllers support online expansion. It is high-risk and takes many hours. Always take a full backup before expanding an array.

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Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How It Works

RAID combines several physical disks into one logical volume, trading raw capacity for speed, redundancy, or both. This calculator applies the standard per-level capacity formulas to your disk count n and per-disk size s: RAID 0 = n x s (pure striping, no protection); RAID 1 = (n / 2) x s (mirroring); RAID 5 = (n - 1) x s (single distributed parity); RAID 6 = (n - 2) x s (double parity); RAID 10 = (n / 2) x s (mirrored then striped); JBOD = n x s (concatenation only).

The calculator also converts usable TB to TiB (multiply by 0.909) since operating systems report binary tebibytes while drive makers sell decimal terabytes. It reports parity overhead, fault tolerance (drives that can fail without data loss), and rough read/write throughput multipliers.

Use Cases

Storage engineers and IT architects use this calculator to:

Compare usable capacity and fault tolerance across RAID levels before purchasing drives for a NAS, SAN, or server.

Determine how many drives to buy to achieve a target usable capacity at a given RAID level.

Explain to stakeholders why RAID 6 on eight 12 TB drives yields only 72 TB usable (75% efficiency) but tolerates two simultaneous failures.

Choose between RAID 5 and RAID 6 for large-capacity drive arrays where rebuild windows increase the risk of a second failure.

Size a RAID 10 array for a database workload that needs both fast random writes and fault tolerance.

Tips

RAID is not a backup - RAID survives drive failure, not accidental deletion, ransomware, controller faults, or site-level disasters. Always maintain a separate offsite backup.

Avoid RAID 5 with large drives - rebuilding a 12 TB+ RAID 5 array after a failure takes many hours during which a second failure destroys everything; use RAID 6 for drives larger than 4 TB.

Mix identical drive models - drives from the same batch of the same model tend to fail together (infant mortality and wear-out); use drives from different manufacturing batches if possible.

Monitor SMART health continuously - a drive showing reallocated sectors or pending uncorrectable errors is likely to fail within weeks; replace it before it becomes a recovery situation.

Test restores from real backups quarterly - the worst time to discover your backup is broken is during a RAID rebuild emergency.

FAQ

What RAID level should I choose for a database?

RAID 10 is the preferred choice for databases and virtual machines. It provides fast reads (all drives contribute), fast writes (no parity calculation penalty), and tolerates one failure per mirrored pair. RAID 5 and 6 suffer a significant write penalty due to the read-modify-write cycle required for parity updates, making them poor choices for random-write workloads.

Why does RAID 6 require at least 4 drives?

RAID 6 uses double distributed parity, requiring two drives worth of parity data. With fewer than 4 drives, storing two parity blocks across the array would leave no usable capacity or violate the distribution requirement. RAID 5 requires at least 3 drives for the same reason with single parity.

What is the TB vs TiB difference in practice?

Drive manufacturers define 1 TB as exactly 10^12 bytes (decimal). Operating systems like Linux, macOS, and Windows define 1 TiB as 2^40 bytes (binary). The ratio is approximately 0.909, meaning an 8 TB RAID array shows as roughly 7.27 TiB in the OS. Both values are correct but in different unit systems.

Can I add drives to an existing RAID array to expand capacity?

Some RAID controllers and software RAID implementations (like mdadm or ZFS) support online expansion by adding drives and reshaping the array. However, this is a high-risk operation that takes many hours, during which a second drive failure would cause data loss. Always take a full backup before expanding an array.

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