raid disk calculator

If you are sizing a NAS, homelab server, backup target, or production storage array, this RAID disk calculator gives you a fast way to estimate how much space you will really have after RAID overhead. Raw disk totals can be misleading. RAID parity, mirroring, spare drives, and filesystem overhead all reduce what is actually available for data.

What this RAID disk calculator tells you

The calculator above focuses on practical planning outputs:

• Raw active capacity: sum of active drives (excluding hot spares).
• Usable RAID capacity: capacity after parity or mirror overhead.
• Estimated formatted capacity: usable capacity minus filesystem/format overhead.
• Storage efficiency: percentage of raw capacity usable for data.
• Fault tolerance: how many drive failures your RAID level can survive.

Quick RAID level guide

RAID 0 (striping)

Maximum performance and full capacity, but no redundancy. If one drive fails, all data in the array is lost. Useful for scratch data or temporary workloads where speed matters more than protection.

RAID 1 (mirroring)

Data is mirrored across drives. Strong protection, but low capacity efficiency. In this calculator, RAID 1 is treated as n-way mirroring, where usable capacity equals one drive.

RAID 5 (single parity)

Good balance of capacity and protection. You lose the equivalent of one drive to parity. Can survive one drive failure. Common for small-to-medium NAS deployments.

RAID 6 (dual parity)

Like RAID 5 but safer for large disks or larger arrays. You lose the equivalent of two drives to parity, and the array can survive two concurrent failures.

RAID 10 (striped mirrors)

Excellent read/write performance and fast rebuilds with good resilience. Capacity is roughly half of active raw capacity. Requires an even number of drives.

Capacity formulas used

The calculator uses standard planning formulas for equal-sized drives:

• RAID 0: usable = N × drive size
• RAID 1: usable = 1 × drive size (n-way mirror model)
• RAID 5: usable = (N − 1) × drive size
• RAID 6: usable = (N − 2) × drive size
• RAID 10: usable = (N / 2) × drive size

Where N is the number of active drives after subtracting hot spares.

Real-world examples

Example 1: 6 × 12 TB, RAID 5, 1 hot spare

Active drives = 5. Usable RAID capacity = (5 − 1) × 12 = 48 TB before formatting overhead. With 5% overhead, you get about 45.6 TB estimated usable.

Example 2: 8 × 8 TB, RAID 6, no spare

Usable capacity = (8 − 2) × 8 = 48 TB. You sacrifice more space than RAID 5, but gain protection against two simultaneous drive failures.

Example 3: 8 × 4 TB, RAID 10

Usable capacity = (8 / 2) × 4 = 16 TB. Lower efficiency than RAID 5/6, but typically better write performance and safer rebuild behavior under heavy load.

How to choose the right RAID level

• Choose RAID 0 only when data is disposable and speed is the sole priority.
• Choose RAID 1 for simple mirrored protection and small arrays.
• Choose RAID 5 when you want better capacity efficiency and can accept single-drive fault tolerance.
• Choose RAID 6 when using large drives or mission-critical data where rebuild risk is a concern.
• Choose RAID 10 when you need strong random I/O performance and predictable rebuild behavior.

Important planning notes

1) RAID is not backup

RAID protects against drive failure, not accidental deletion, ransomware, filesystem corruption, or site-level disasters. Always keep independent backups.

2) Rebuild windows matter

Larger drives can take many hours or days to rebuild, increasing exposure to another failure. This is a key reason many teams favor RAID 6 over RAID 5 on large-capacity arrays.

3) Mixed drive sizes reduce effective capacity

Most RAID implementations treat all drives as the size of the smallest disk in the array. For accurate planning, use matching models and firmware where possible.

4) Controller and filesystem details can change real results

This calculator gives a practical estimate. Real-world usable space can vary based on metadata, block size, reserved space, thin provisioning policy, and vendor implementation details.

FAQ

Does this calculator work for Synology, QNAP, TrueNAS, and hardware RAID?

Yes, for rough planning. Vendor-specific systems may add metadata overhead or support hybrid layouts that alter exact totals.

Why show both decimal and binary units?

Drives are usually marketed in decimal TB/GB, while operating systems often display binary TiB/GiB. Seeing both helps avoid surprises after deployment.

Should I configure hot spares?

For important arrays, yes. A hot spare can reduce time spent in degraded mode by starting rebuilds automatically after a disk failure.