How do I calculate usable RAID capacity?

Usable capacity depends on the RAID level. RAID 5 = (N−1) × D, RAID 6 = (N−2) × D, RAID 10 = (N÷2) × D, where N is the number of drives and D is the per-disk capacity (the smallest disk if they differ). Filesystem overhead then reduces the formatted volume slightly.

Which RAID level should I use?

RAID 1 or 10 for boot/OS and databases, RAID 5 for small NAS and file servers, RAID 6 for large archive arrays where a second failure during rebuild is a real risk, and RAID 0 or JBOD only for scratch data you can afford to lose.

How long does a RAID rebuild take?

Rebuild time ≈ drive capacity ÷ effective rebuild speed. A 4 TB drive at ~140 MB/s effective throughput rebuilds in roughly 8 hours; higher live I/O load during the rebuild lengthens it and widens the window in which a second failure is fatal.

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RAID Calculator

Calculate usable space, efficiency and fault tolerance for RAID 0, 1, 5, 6, 10, 50, 60 and JBOD — then compare every level and estimate rebuild time. Everything runs in your browser; nothing is uploaded.

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Disk configuration

Number of disks

Disk size

Display units

FS overhead: 5%

RAID level

RAID 5

Single distributed parity. One disk of capacity is spent on parity; tolerates one failure.

Usable space

19 TB

17.28 TiB on-disk

Fault tolerance

1 drive

1-drive safety

Efficiency

83%

24 TB raw

Storage lost

5 TB

parity + overhead

Usable 79%Parity / mirror 17%FS overhead 4%

During a rebuild, a second drive failure causes total data loss. For large arrays (≥ 6 disks or ≥ 8 TB drives), prefer RAID 6.

Rebuild time estimator

Rebuild speed200 MB/s

I/O load during rebuild30%

7.94 h

to rebuild one 4 TB drive

0.3

days · 140 MB/s effective

The array stays online during rebuild but runs degraded. Higher live I/O load slows reconstruction and widens the danger window.

Compare every RAID level

RAID level	Usable space	Efficiency	Fault tolerance	Min drives	Read	Write
RAID 0	22.8 TB	100%	None	2	Max	Max
RAID 1	3.8 TB	17%	5 drives	2	Great	Good
RAID 5	19 TB	83%	1 drive	3	Great	Good
RAID 6	15.2 TB	67%	2 drives	4	Great	Good
RAID 10	11.4 TB	50%	1 per pair	4	Max	Great
RAID 50	15.2 TB	67%	1 per group	6	Max	Good
RAID 60	—	—	2 per group	8	Max	Good
JBOD	22.8 TB	100%	None	1	Normal	Normal

Click any row to make it the active level above. Greyed rows need more drives than your current 6-disk configuration.

Quick reference cheat sheet

At-a-glance guidance for choosing a RAID level by workload.

Level	Space	Redundancy	Performance	Best for
RAID 0	100%	None	⚡ Max	Scratch, render, temp
RAID 1	1 / N	N−1 drives	↗ Great	Boot, OS, critical data
RAID 5	(N−1) / N	1 drive	↗ Great	NAS, files, servers
RAID 6	(N−2) / N	2 drives	↗ Great	Archives, large arrays
RAID 10	1 / 2	1 per mirrored pair	⚡ Max	Databases, VMs, critical
RAID 50	(N−G) / N	1 per group	⚡ Max	Large high-performance arrays
RAID 60	(N−2G) / N	2 per group	⚡ Max	Enterprise archives
JBOD	100%	None	→ Normal	Bulk storage, no perf need