How to use the raid capacity calculator
- Choose the RAID level: 0, 1, 5, 6 or 10.
- Enter the number of drives and the size of each drive in GB or TB. If your drives differ, enter the smallest, since the array uses the smallest size on every drive.
- Read the usable capacity in TB and in TiB, the space given up to redundancy and the efficiency.
- Check how many drive failures the level survives, and download the CSV if you need to share the numbers.
Formula
RAID 0 usable = n x drive size
RAID 1 usable = 1 x drive size (every drive holds a full copy)
RAID 5 usable = (n - 1) x drive size; RAID 6 usable = (n - 2) x drive size
RAID 10 usable = n / 2 x drive size, with an even number of drives
TiB = bytes / 2^40; TB = bytes / 10^12
Drive makers count in powers of ten, where 1 TB is one trillion bytes. Many operating systems count in powers of two and show the same space as fewer TiB, so a 24 TB array shows up as about 21.8 TiB. Nothing is lost; it is a difference in units.
Worked example
Four 8 TB drives hold 32 TB raw. In RAID 5 one drive of space goes to parity, so usable capacity is 3 x 8 = 24 TB, or 21.83 TiB, which is 75% efficiency, and one drive can fail.
The same four drives in RAID 6 give 16 TB and can lose any two drives. In RAID 10 they also give 16 TB, with a mirror pair on each side, and in RAID 1 only 8 TB. RAID 0 offers all 32 TB, but a single failure loses everything.
Three 500 GB drives in RAID 5 give 2 x 500 GB = 1 TB.
Choosing between the levels
Each level trades capacity for protection and speed.
- RAID 0 splits data across drives for speed and capacity and has no protection, so one failed drive loses the whole array.
- RAID 1 mirrors data, so it keeps working if all but one drive fail, but it gives the space of one drive.
- RAID 5 survives one failure and is efficient with many drives, though rebuilding a large drive can take a long time.
- RAID 6 survives two failures and is often preferred for large drives.
- RAID 10 combines mirroring and striping for good speed and survives a failure in each mirror pair, at half the capacity.
What RAID does not do
RAID protects against a drive failing. It does not protect against deleting a file by mistake, malware, a failed controller or a fire. A real backup is a separate copy of your data, ideally in another place. Treat RAID as a way to stay online through a failure and keep backups as well.
File system overhead, hot spare drives and reserved space reduce usable capacity further, so leave some margin when planning.
Assumptions and limits
- All drives are the same size. With mixed drives, enter the smallest and expect the remainder to be unused.
- Capacity is calculated at the block level and does not subtract file system overhead, spare drives or reserved space.
- Fault tolerance is the guaranteed number of failures. RAID 10 can survive more when failures fall in different mirror pairs.
- Only levels 0, 1, 5, 6 and 10 are included. Nested and vendor specific levels are not.
Frequently asked questions
How much usable space do I get with RAID 5?
You lose the capacity of one drive to parity. With four 8 TB drives, the usable space is 24 TB. The more drives in the array, the smaller the share given to parity, so efficiency rises as you add drives.
Why does my array show less space than the calculator?
Operating systems often show space in TiB, which are larger units than TB. A 24 TB array appears as about 21.8 TiB. File system overhead takes a little more.
What is the difference between RAID 6 and RAID 10?
RAID 6 gives the space of n minus two drives and survives any two failures. RAID 10 gives half the raw space and is faster to rebuild and write, but can fail if both drives in one mirror pair are lost.
Can I mix drive sizes in RAID?
Most standard setups use the size of the smallest drive on every drive, so extra space on larger drives goes unused. Enter the smallest size here to see what the array will give you.