Does this include hot spares?

No. Add spares separately by reducing the disk count available to the array.

What if my drives are different sizes?

Mixed drives are limited by the smallest disk in most RAID levels. Use the smallest size for a conservative estimate.

Does this account for filesystem or formatting overhead?

No. File systems, block sizes, and metadata reduce usable space further—plan a few percent of overhead.

No. RAID protects against drive failure, not deletion or corruption. Keep separate backups.

How accurate is RAID 10 fault tolerance?

It assumes one drive can fail per mirror pair. Real tolerance depends on which specific disks fail.

tech calculator

RAID Capacity Calculator

Estimate usable storage and drive fault tolerance for common RAID levels.

Results

Usable capacity (TB): 50.00
Drive failures tolerated: 1.00

How to use this calculator

Enter the number of disks and their raw size.
Select a RAID level (0, 1, 5, 6, or 10).
See usable capacity and how many disk failures you can tolerate.

Inputs explained

Disk count: Total drives in the array (exclude hot spares here).
Disk size (TB): Raw capacity per disk; mixed sizes should use the smallest drive.
RAID level: Protection scheme determining parity/mirroring and fault tolerance.

How it works

Different RAID levels reserve disks for parity or mirroring.

Usable disks are based on the level; usable TB = usable disks × disk size.

Formula

RAID0: N
RAID1: N/2
RAID5: N−1
RAID6: N−2
RAID10: N/2

When to use it

Sizing a NAS or server build to balance capacity and redundancy.
Explaining capacity trade-offs to stakeholders before purchasing disks.
Comparing RAID 5 vs RAID 6 vs RAID 10 for rebuild risk and usable space.

Tips & cautions

If disks are mismatched, capacity is capped by the smallest drive—enter that size for an accurate estimate.
Consider adding hot spares separately; they reduce initial usable capacity but speed rebuilds.
Large arrays with big disks may prefer RAID 6/10 to reduce rebuild risk versus RAID 5.
Simplified math—does not include hot spares, filesystem overhead, or vendor-specific layouts.
RAID 10 fault tolerance depends on which disks fail; we show typical 1-disk-per-mirror assumption.
Rebuild times, URE risk, and performance are not modeled here.

Worked examples

6×10 TB, RAID 6

Usable ≈ 40 TB
Tolerates 2 failures

8×8 TB, RAID 10

Usable ≈ 32 TB
Tolerates 1 failure (per mirror set)

Deep dive

This RAID capacity calculator shows usable storage and drive fault tolerance for RAID 0, 1, 5, 6, and 10. Enter disk count, disk size, and level to see how much space remains after parity or mirroring.

Use it to choose the right setup for a NAS or server and to communicate redundancy trade-offs. Account for hot spares and filesystem overhead separately when finalizing capacity plans.

FAQs

Does this include hot spares?: No. Add spares separately by reducing the disk count available to the array.
What if my drives are different sizes?: Mixed drives are limited by the smallest disk in most RAID levels. Use the smallest size for a conservative estimate.
Does this account for filesystem or formatting overhead?: No. File systems, block sizes, and metadata reduce usable space further—plan a few percent of overhead.
Is RAID a backup?: No. RAID protects against drive failure, not deletion or corruption. Keep separate backups.
How accurate is RAID 10 fault tolerance?: It assumes one drive can fail per mirror pair. Real tolerance depends on which specific disks fail.

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