MotionShine01 / Capacity
Usable space
Multiply the disks by the size, then take away the space that holds copies or parity.
Free tool
RAID calculator, usable space and safety.
Pick a RAID level, type how many disks you have and how big each one is, and this RAID calculator shows the usable capacity, how much space goes to redundancy and how many disks can fail. It shows both TB and TiB, compares RAID 0, 1, 5, 6, 10 and JBOD side by side for the same disks, and tells you plainly when a level needs more disks. It is maths only, and RAID is not a backup.
RAID keeps a disk failure from stopping you. It does not protect against deleted files, a virus, a bad update, a fire or a failed controller. Keep a separate backup.
Disks sold as TB use 1,000,000,000,000 bytes. Many systems show TiB, which is 1,099,511,627,776 bytes. Hot spares sit idle until a disk fails and are not counted in the capacity.
Worked out in your browser. Nothing is sent anywhere.
The maths
Disks times size, minus what protects it.
02 / Safety
Fault tolerance
How many disks can fail at once before the data is gone, for each level.
03 / Units
TB and TiB
The same capacity in decimal and binary units, so it matches what your system shows.
Guide
How to calculate RAID capacity, level by level.
A RAID calculator tells you how much space you can use when several disks are joined into one volume, and how many of them can fail before you lose data. The answer depends on the RAID level. RAID 0 gives you every byte but no safety. RAID 1 mirrors, so you get the space of one disk. RAID 5 and RAID 6 give up one and two disks of space for parity, and RAID 10 gives up half. This calculator works out the numbers for RAID 0, 1, 5, 6, 10 and JBOD from the number and size of your disks, and shows all of them side by side.
How to use the RAID calculator
- Choose a level under RAID level.
- Type the Number of disks and the Size of each disk, and pick GB, TB, GiB or TiB under Unit.
- If you keep a spare disk waiting, type it under Hot spares. Spares are taken out of the set and do not add to the capacity.
- Optionally type Capacity you need. The table then shows how many disks each level needs to reach it.
- Read the cards for your level, and the table for every level. Copy result copies a line of text and Copy link a link with your numbers.
Usable capacity by level
In these formulas, n is the number of disks in the set and s is the size of one disk.
| Level | Min disks | Usable | Disks that can fail |
|---|---|---|---|
| JBOD | 1 | n × s | 0 |
| RAID 0 | 2 | n × s | 0 |
| RAID 1 | 2 | s | n − 1 |
| RAID 5 | 3 | (n − 1) × s | 1 |
| RAID 6 | 4 | (n − 2) × s | 2 |
| RAID 10 | 4, an even number | (n ÷ 2) × s | 1 for certain, up to n ÷ 2 |
RAID 10 can survive more than one failure only when the failed disks are in different mirrored pairs. If both disks of one pair fail, the set is lost. In RAID 1 the page treats every disk as a full copy, which is how a two-disk mirror works. Some controllers allow three or more copies.
Worked examples
Four 4 TB disks. The raw space is 16 TB. RAID 5 gives (4 − 1) × 4 = 12 TB, which is 75 percent. RAID 6 gives (4 − 2) × 4 = 8 TB, which is 50 percent, and RAID 10 gives (4 ÷ 2) × 4 = 8 TB, also 50 percent. RAID 0 gives all 16 TB.
Six 8 TB disks. The raw space is 48 TB. RAID 5 gives 5 × 8 = 40 TB, and RAID 6 gives 4 × 8 = 32 TB. Going from RAID 5 to RAID 6 costs one more disk of space and lets a second disk fail.
Needing 20 TB from 8 TB disks. 20 ÷ 8 is 2.5, so you need 3 disks of data. RAID 5 adds one for parity, so 4 disks give 24 TB. RAID 6 adds two, so 5 disks. RAID 10 doubles the 3 to 6.
Why 12 TB shows up as 10.91 TiB
Disk makers count in decimal: 1 TB is 1,000,000,000,000 bytes. Many operating systems count in binary: 1 TiB is 1,099,511,627,776 bytes, and some label it TB anyway. So 12 TB is 12,000,000,000,000 ÷ 1,099,511,627,776 = 10.91 TiB. A single 4 TB disk is 3.64 TiB. Nothing is missing. The two units simply measure with different rulers, and this page shows both. On top of that, the file system and the RAID metadata take a little space of their own.
Mixed disk sizes
A RAID set uses the smallest disk's size from every disk. If you mix a 4 TB and an 8 TB disk in a mirror, you get 4 TB. Type the smallest size and the real disk count, and the answer holds. The extra space on the larger disk is not used by the set.
RAID is not a backup
RAID keeps the volume running when a disk fails. It copies your mistakes at the same speed as your files, so a deleted folder, ransomware or a bad update is on every disk a moment later. A fire, a theft or a failed controller can take every disk at once. Keep a backup on a different device, and ideally another place. A second failure while a RAID 5 set is rebuilding loses the set, which is one reason RAID 6 and RAID 10 exist.
Questions
Good to know.
How do I calculate RAID capacity?
Multiply the number of disks by the disk size, then remove the redundancy. RAID 5 loses one disk of space, RAID 6 two, RAID 10 half, and RAID 1 keeps one disk's worth. The table above lists each formula.
How much usable space does RAID 5 give?
(n − 1) × the disk size. Four 4 TB disks give 12 TB. It needs at least 3 disks and survives one failed disk.
What is the difference between RAID 5 and RAID 6?
RAID 6 keeps two disks of parity, so it survives two failed disks but gives up one more disk of space. It needs at least 4 disks.
RAID 10 or RAID 5?
RAID 10 gives half the raw space and survives at least one failure, more if they are in different pairs. RAID 5 gives more space for the same disks but survives only one. Pick on how much space and how much safety you need.
Why is my RAID smaller than the disks added together?
Redundancy takes some space, and TB and TiB are different units. 12 TB is 10.91 TiB. The calculator shows both.
How many disks do I need for RAID 5, 6 or 10?
At least 3 for RAID 5, 4 for RAID 6, and 4 for RAID 10, which also needs an even number. The page says so if you enter fewer.
Is RAID a backup?
No. RAID only keeps running through a disk failure. It does not protect against deleted files, malware, fire or a controller fault. Keep a separate backup.
What is a hot spare?
An extra disk that sits unused until a disk fails, then takes over and starts the rebuild. It is not counted in the usable capacity.
What is JBOD?
Just a bunch of disks. The disks are joined or listed together with no copies, so you get every byte and no protection.