Does RAID replace backups?

No. RAID can help with drive failure tolerance, but it does not replace backups. Deleted files, ransomware, theft, fire, controller failure, and bad rebuilds still require separate backups.

Does this support mixed drive sizes?

This first version assumes same-size drives. For standard RAID with mixed drives, usable capacity is often constrained by the smallest drive, but vendor-specific layouts can behave differently.

Why is usable capacity lower than raw capacity?

RAID levels use capacity for parity, mirroring, or other redundancy. Filesystems, snapshots, hot spares, and vendor reserves can reduce usable capacity further.

Free IT storage calculator

RAID Capacity Calculator

Estimate usable storage, raw capacity, parity or mirror overhead, reserve overhead, and basic fault tolerance for common RAID layouts.

RAID 0/1/5/6/10
JBOD/no RAID
GB, TB, GiB, TiB
Estimate-only storage math

Capacity result

Estimated usable storage

RAID 5 estimated usable capacity0 TB

Enter drive details to estimate capacity.

Raw drive capacity: 0 TB
Usable before reserve: 0 TB
Usable after reserve: 0 TB
RAID parity or mirror overhead: 0 TB
Reserved overhead: 0 TB
Fault tolerance: 1 drive
Minimum drives: 3 drives
Layout note: Estimate only.

RAID is not backup. Keep separate backups for important data.

How the RAID capacity calculator works

The calculator multiplies drive count by drive size to estimate raw capacity, then applies the selected RAID layout. RAID 0 and JBOD use all entered capacity, RAID 1 mirrors data, RAID 5 reserves one drive worth of parity, RAID 6 reserves two drives worth of parity, and RAID 10 uses mirrored pairs.

The optional reserved overhead field subtracts an additional planning cushion for filesystem metadata, snapshots, hot spare policy, free-space targets, or practical headroom.

Same-size drive assumption

This first version assumes every drive has the same usable size. If you mix drive sizes in standard RAID, capacity is often limited by the smallest drive. For a conservative estimate, enter the smallest drive size.

Some NAS platforms offer vendor-specific mixed-drive layouts that can use more capacity than traditional RAID. Treat this page as a planning estimate before checking your vendor's final storage pool calculator.

What each RAID level means

RAID 0 stripes data and gives high capacity, but one failed drive can break the array. RAID 1 mirrors data and is simple, but capacity is limited to one drive in a same-size mirror. RAID 5 and RAID 6 use parity to recover from one or two failed drives. RAID 10 mirrors pairs and stripes across those pairs.

Fault tolerance does not protect against accidental deletion, malware, fire, theft, controller problems, bad updates, or multiple failures outside the layout's limits. Use RAID for availability, not as your only copy of important files.

RAID level Usable capacity pattern Good planning use

RAID 0 All drives combined Temporary scratch space where speed matters more than redundancy.

RAID 1 One drive worth in a simple mirror Small systems where simplicity and quick recovery matter.

RAID 5 Total minus one drive Capacity-efficient arrays with one-drive fault tolerance.

RAID 6 Total minus two drives Larger arrays where rebuild risk makes extra parity worthwhile.

RAID 10 About half of total capacity Performance-focused storage that still needs mirrored redundancy.

RAID capacity FAQ

Why does my NAS show less space? The NAS may use binary units, metadata, snapshots, filesystem reserves, hot spares, or a vendor-specific storage layout.

Should I use TB or TiB? Use TB for decimal drive labels and TiB when comparing against operating-system or NAS screens that report binary capacity.