Four 4 TB drives do not give you 16 TB once they are in an array. RAID 5 gives you 12, RAID 6 gives you 8, and the screen then shows less again because it counts in a different base. This RAID calculator does both sums and shows each one.
Pick the level, the number of drives and the size printed on them. It works out the space left for your files, what Windows or a NAS will report, and how many drives can fail before anything is lost. Every figure it supplies names its source, and the list of what it cannot tell you is below the answers.
In this guide
The RAID calculator
The level you plan to set up in the NAS or the controller.
Every drive in the array. A fraction is counted down to whole drives.
The size printed on the drive. If the drives differ, put the smallest: a standard array uses only that much of every drive.
How that is worked out, line by line
- Whole drives counted4
- All the drives added up16.00 TB
- Drives' worth that hold your data3
- Space given to protection4.00 TB
- Share of the raw space you can use75 %
On those numbers you are in the One drive can fail band, which is the section below.
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Where every number here comes from
No part of this was measured on a bench here. This site works from published specifications and from what owners report, and every number below is either one you typed in or a published figure with its source named next to it.
The figures this page supplies
- Drives' worth RAID 5 gives to parity: 1. RAID 5 stores one parity block for each stripe, spread across all the drives, which costs one drive's worth of space in total. Patterson, Gibson and Katz defined it this way in 1988, and SNIA's Common RAID Disk Data Format specification carries the same definition.
- Drives' worth RAID 6 gives to parity: 2. RAID 6 stores two independent parity blocks for each stripe, which costs two drives' worth of space and is why it survives two failures. SNIA's Common RAID Disk Data Format specification.
- Copies RAID 10 keeps of every block: 2. Classic RAID 10 is a stripe across mirrored pairs, so every block is written twice and half the raw space holds data. Seagate's and Synology's own RAID documentation both put it at half.
- Fewest drives for RAID 0: 2. Seagate's NAS OS manual: striping needs at least two drives.
- Fewest drives for RAID 1: 2. A mirror needs at least two drives to protect anything (Seagate's NAS OS manual).
- Fewest drives for RAID 5: 3. Seagate's NAS OS manual and Synology's RAID documentation: three drives.
- Fewest drives for RAID 6: 4. Seagate's NAS OS manual and Synology's RAID documentation: four drives.
- Fewest drives for RAID 10: 4. Two mirrored pairs, so four drives (Seagate's NAS OS manual and Synology's RAID documentation). It also needs an even count, since every drive has a partner.
- What one advertised TB shows as: 0.9095 TB as Windows shows it. Drive makers count a terabyte as 10^12 bytes. Windows and most NAS screens count 2^40 bytes and still call it TB, which IEC 80000-13 names a TiB. 10^12 / 2^40 is 0.9095.
Why each step is the step it is
- Whole drives counted:
Math.floor(drives). A drive is either in the array or it is not, so a fraction is counted down. - All the drives added up:
n * drive_tb. The drive count times the size of each, as printed on the box. This is what RAID 0 gives you and what every other level spends part of on protection. - Drives' worth that hold your data:
is_r0 * n + is_r1 + is_r5 * (n - r5_parity) + is_r6 * (n - r6_parity) + is_r10 * n / r10_copies + valid. RAID 0 uses every drive. RAID 1 writes the same data to every drive, so it holds one drive's worth however many there are. RAID 5 gives one drive's worth to parity and RAID 6 two. RAID 10 keeps two copies of everything, so half the drives' worth holds data. - Space for your files, as drives are sold:
data_drives * drive_tb. The drives' worth that hold data, times the size of each drive. - The same space as Windows or a NAS shows it:
usable_tb * tib_per_tb. The decimal terabytes on the box converted to the binary units the screen counts in. Nothing is lost: the two are counting in different bases. - Drives that can fail without losing data, whichever they are:
is_r1 * (n - 1) + is_r5 * r5_parity + is_r6 * r6_parity + is_r10 + valid. RAID 0 survives none. A RAID 1 mirror survives all but one of its drives. RAID 5 survives one and RAID 6 two, one per drive's worth of parity. RAID 10 is counted at one because that is all it can promise: a second failure is survived only when it lands in a different mirrored pair. - Space given to protection:
raw_tb - usable_tb. What the drives hold in total, less what is left for your files. - Share of the raw space you can use:
usable_tb / raw_tb * 100. Usable space divided by raw space. RAID 5 on four drives is 75%; mirroring is 50% at best.
What this cannot tell you
Every calculator has an edge, and a page that hides its edge is worse than no page at all. Here is this one's.
- It is the arithmetic of the RAID levels and nothing else. No drive or array was measured for it, and this site has never had any of these drives in its hands.
- RAID is not a backup. It keeps an array running through a failed drive, but a deleted file, ransomware, a failed enclosure or a fire reaches every copy at once. Keep a separate copy somewhere else as well.
- A NAS takes its own share before you see any of this: Synology, for one, keeps about 10 GB of every drive for its system, and the file system keeps back a few percent more.
- Mixed drive sizes are counted at the smallest drive, which is what standard RAID does. Schemes built to use the leftover space, such as Synology's SHR, are not calculated here.
- The failure count is the worst case. RAID 10 can survive more than one failure when they land in different pairs, and loses everything when both drives of one pair fail. Linux software RAID 10 also runs on an odd number of drives, which this paired layout does not cover.
- It says nothing about speed, rebuild time or whether a drive is built for running in an array. Check that the drive's maker lists it for NAS or RAID use.
What that protection means for you
These follow from the answer above and nothing else. Each heading below covers one span of results, so read the one the calculator pointed you at and ignore the rest.
No drive can fail
RAID 0 adds the drives together and protects nothing: when one drive fails, the whole array goes with it. Only keep data on it that has a full copy somewhere else, such as a separate external drive.
HDD or SSD for Games and Backups Worth Buying 2026 covers this range in full.
One drive can fail
RAID 5, RAID 10 at worst, or a two-drive mirror. The array keeps running through one failure and is exposed until the replacement has rebuilt. It is still not a backup: a deleted file or ransomware reaches every drive at once, so keep a separate copy. This site has not covered NAS drives or enclosures yet, so it is not going to name one; check the drive maker lists a drive for NAS or RAID use before buying it for an array.
HDD or SSD for Games and Backups Worth Buying 2026 covers this range in full.
SSD vs HDD in 2026: Which Drive Is Worth Your Money covers this range in full.
Two or more drives can fail
RAID 6, or a mirror across three or more drives. A second failure during a rebuild is the case this protects against, and it costs more drives' worth of space to do it. The same rule holds: an array is not a backup, so keep a copy on a separate drive.
HDD or SSD for Games and Backups Worth Buying 2026 covers this range in full.
That array cannot be built
Each level has a smallest number of drives: 2 for RAID 0 and RAID 1, 3 for RAID 5, 4 for RAID 6, and 4 or more in pairs for RAID 10. Add a drive or pick a different level and the calculator will answer.
If you are still deciding how big each drive should be, the storage calculator adds up your own files first. Then come back here with that figure and see which level gets you there with the protection you want.
Questions about the RAID calculator
How much usable space does RAID 5 give?
All the drives but one, because RAID 5 spends one drive's worth of space on parity. With 4 drives of 4 TB it gives 12 TB of the 16 TB raw, which is 75%, and Windows or a NAS shows that as 10.91 TB. With 8 of the same drives it gives 28 TB, 88% of the raw space, because the parity costs one drive's worth however many there are. RAID 5 needs at least 3 drives. The RAID calculator above does the same sum for any count and size, and for RAID 0, 1, 6 and 10 as well.
How many drives can fail in RAID 5, RAID 6 and RAID 10?
RAID 5 survives 1 failed drive and RAID 6 survives 2, one for each drive's worth of parity. RAID 10 is counted at 1, because that is all it can promise: a second failure is survived only when it lands in a different mirrored pair, and losing both drives of one pair loses the array. A 2-drive RAID 1 mirror survives 1, all but one of its drives, and RAID 0 survives 0. The RAID calculator gives the worst case, which is the number to plan around.
RAID 5 or RAID 6: what does the extra protection cost?
One more drive's worth of space. With 4 drives of 4 TB, RAID 5 gives 12 TB and RAID 6 gives 8 TB. With 8 of the same drives the cost is still one drive, but a smaller share of the whole: 28 TB against 24 TB. What RAID 6 buys with it is surviving a second failure, including one during the rebuild after the first, which is when an array with one drive's worth of parity has no protection left.
Why does my NAS show less space than the RAID calculator?
Two reasons, and the RAID calculator shows the first. Drive makers count a terabyte as 10^12 bytes, while Windows and most NAS screens count 2^40 bytes and still write TB, so 12 TB on the box shows as 10.91 TB on screen; nothing is lost, the two count in different bases. The second is what the NAS keeps for itself: Synology, for one, keeps about 10 GB of every drive for its system, and the file system keeps back a few percent more. This page does not subtract that, because it is not the same on every NAS.
Is RAID a backup?
No. RAID keeps an array running through a failed drive, but a deleted file, ransomware, a failed enclosure or a fire reaches every copy at once. Even RAID 6, which survives 2 failed drives, holds no copy anywhere else. Keep a separate copy of anything that matters on another drive, in another place.
How many drives do I need for RAID 5, RAID 6 or RAID 10?
At least 3 for RAID 5, 4 for RAID 6 and 4 for RAID 10, which also needs an even number, because every drive has a mirror partner. RAID 0 needs 2 and RAID 1 needs 2. Put in fewer and the RAID calculator refuses to give a capacity rather than describe an array nobody can build. Linux software RAID 10 can run on an odd count, which this paired layout does not cover.
Sources
- Patterson, Gibson and Katz: A Case for Redundant Arrays of Inexpensive Disks (RAID), UC Berkeley: The paper that defined the RAID levels, including the one-drive parity cost of RAID 5 and the full-copy cost of mirroring.
- SNIA Common RAID Disk Data Format specification: The industry standard for how the RAID levels, RAID 6's double parity among them, are laid out across drives.
- Seagate NAS OS manual: RAID modes: A drive maker's own statement of each level's minimum drive count, capacity and failures survived.
- Synology: choose a RAID type, and the Synology RAID calculator: A NAS maker's figures for the same levels, and the space its own system and file system keep back from every array, which this page does not subtract.
- Linux md(4) manual page: Shows that Linux software RAID 10 can run on an odd number of drives, which this page's classic paired layout does not cover.
- IEC 80000-13, prefixes for binary multiples: Defines the binary units a screen counts in, which is why an array shows less than the box says.
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