Free calculator · no sign-up

RAID Calculator Usable space for RAID 0, 1, 5, 6 and 10

Pick a RAID level, the number of drives and their size. You get usable capacity, the space given up for protection and how many drives can fail.

Five common RAID levels
2 to 24 drives, sized in TB or GB
Flags layouts a level can’t build

Built by Marcus Reed, laptop and desktop repair technician

RAID calculatorRAID 0 · 1 · 5 · 6 · 10

RAID level

Number of drives2 to 24

Size of each driveWith mixed sizes, enter the smallest

Example

Usable capacity

–

Capacities are in decimal terabytes as printed on the drive label. Your operating system or NAS may show a smaller number in binary units (TiB), and file systems reserve some space too.

24 TBusable from four 8 TB drives in RAID 5
2 drivescan fail in RAID 6 without data loss
50%of raw space kept by RAID 10
about 9%smaller when a NAS shows TiB instead of TB

Quick answer

Usable RAID capacity depends on the level: RAID 0 uses every drive, RAID 1 keeps one drive of space, RAID 5 loses one drive to parity, RAID 6 loses two, and RAID 10 keeps half. Four 8 TB drives give 32 TB in RAID 0, 24 TB in RAID 5, 16 TB in RAID 6 or RAID 10, and 8 TB in a four-way RAID 1 mirror. Pick your setup in the calculator above.

01 / 05

RAID capacity formulas

For n drives of size s (the smallest drive in the set):

Level	Usable capacity	Minimum drives	Drives that can fail
RAID 0	n × s	2	0
RAID 1	s	2	n − 1
RAID 5	(n − 1) × s	3	1
RAID 6	(n − 2) × s	4	2
RAID 10	(n ÷ 2) × s	4, even	1 guaranteed, more if spread across pairs

Some NAS makers also sell their own flexible layouts that use mixed drive sizes more fully. Those follow the maker’s own rules, so check the maker’s calculator for them.

02 / 05

Which RAID level fits a home or small office

Two drives: RAID 1. Half the raw space, and either drive can fail. This is the usual choice for a two-bay NAS holding family photos or office files.

Three or four drives: RAID 5 gives the most space with one-drive protection. RAID 10 with four drives gives half the space, faster writes and quicker rebuilds, since a rebuild copies one mirror instead of reading every drive.

Five or more large drives: RAID 6. Rebuilding a big array takes a long time, and RAID 6 can lose a second drive during that rebuild without losing data.

RAID 0 only makes sense for scratch space you can lose, such as a video editing cache, because one failed drive takes everything with it. An r/DataHoarder regular summed it up: “the number represents the number of files you get back if it breaks.”

All-SSD arrays are the exception to the RAID 6 advice. That advice is about long rebuilds on large hard drives; SSDs rebuild far faster, which is why some r/DataHoarder members argue RAID 5 is reasonable for a high-quality SSD array. Either way, the array still needs a backup.

03 / 05

What your NAS will actually show

The calculator gives usable space in decimal terabytes, the way drives are labeled. Your NAS dashboard will show a smaller number for three reasons, and you can work each one out.

Binary units. Drive makers count 1 TB as 1,000,000,000,000 bytes. Many operating systems and NAS screens count in TiB, where 1 TiB is 1,099,511,627,776 bytes, so the same space reads about 9% smaller. The calculator now shows both.
System space on each drive. Synology’s RAID calculator notes that each drive in the array reserves about 10 GB for the system.
File system overhead. The same Synology page says Btrfs volumes reserve 4% of capacity for metadata and ext4 volumes reserve 2%.

Worked through for four 8 TB drives in RAID 5 on a Synology box:

Step	Space left
Calculator result, RAID 5 with four 8 TB drives	24 TB
Minus about 10 GB of system space on each drive	about 23.97 TB
Counted in binary units	about 21.80 TiB
Minus 4% for a Btrfs volume	about 20.93 TiB
Or minus 2% for an ext4 volume	about 21.36 TiB

So a dashboard reading close to 21 TB on that array is normal, not three missing terabytes. Snapshots, if you turn them on, use more of the same pool over time. Other NAS systems reserve different amounts, so check your maker’s notes for their own figures; Synology’s RAID calculator lists the ones above.

04 / 05

A drive failed: what to do, in order

When one drive dies in RAID 1, 5, 6 or 10, the array keeps working in a degraded state. Synology’s help on repairing a storage pool describes the same thing: the pool stays accessible and no data has been lost yet. What you do next decides whether that stays true.

Copy what you can’t lose, now. If your last backup is old, copy the most important folders off the NAS before anything else. A degraded RAID 5 has no protection left, and a second failure during the repair loses the array.
Find the right drive. Match the failed drive’s bay and serial number in the NAS storage screen before pulling anything. Pulling a healthy drive from a degraded RAID 5 takes the whole array down.
Buy a replacement that fits the rules. For RAID 1, 5, 6 and 10, Synology requires the replacement to be at least as large as the smallest drive in the pool, and of the same type: a hard drive for a hard drive, an SSD for an SSD. Its SHR layout has its own rules, so check the repair page for your version first.
Replace one drive at a time, then run the repair. Use the repair steps the NAS itself shows. When a degraded pool came up on r/synology, the top reply was blunt: “Just follow the instructions from DSM.” Start the rebuild and keep heavy jobs, such as big copies or backups, off the NAS until it finishes.

How long the array stays unprotected depends on how fast the new drive can be filled. A traditional rebuild writes the whole replacement drive, so drive size divided by the drive’s sustained speed gives the shortest possible time. The NAS keeps serving files during the rebuild, so real repairs usually run longer than this:

Drive size	At a steady 150 MB/s	At a steady 250 MB/s
4 TB	7.4 hours	4.4 hours
8 TB	14.8 hours	8.9 hours
16 TB	29.6 hours	17.8 hours
20 TB	37 hours	22.2 hours

A day or more without protection on big drives is the practical reason RAID 6 is worth its second parity drive in larger arrays. Your drive’s data sheet lists its sustained transfer rate if you want your own figure.

05 / 05

RAID keeps you running; backups get you back

A RAID array protects against a dead drive. It does nothing about a deleted folder, ransomware that encrypts every share, or a power surge that takes out the whole box. For an office, the array is one line in a wider plan, and the small business cybersecurity checklist covers the backup and recovery side. For a home NAS, an off-site copy through cloud backup covers what RAID can’t.

FAQ

Questions people ask

How much usable space does RAID 5 give with four 8 TB drives?

24 TB. RAID 5 gives up one drive’s worth of space to parity, so four 8 TB drives leave three drives of usable space, and the array survives one drive failure.

Is RAID 6 better than RAID 5?

RAID 6 survives two failed drives instead of one, and pays for it with a second drive of capacity. With large drives and arrays of five or more, the second parity drive is the safer default, because a rebuild after one failure reads every remaining drive.

Is RAID a backup?

No. RAID keeps the array running when a drive dies. It does not protect against deleted files, ransomware, a failed controller, theft or a fire, because the mistake or damage happens on every drive at once. Keep a separate backup.

Can I mix drive sizes?

In standard RAID 1, 5, 6 and 10, every drive is treated as the size of the smallest one, and the extra space on larger drives goes unused. That is why the calculator asks for the smallest size.

All calculators