RAID Size Calculator
Enter how many drives you have, the size of each and the RAID level. The calculator shows the usable space, how much is lost to parity or mirroring, how many drives can fail and the expected speed.
- Usable space
- Fault tolerance
- TB and TiB
All drives in an array should be the same size. Larger drives only use the space of the smallest one.
RAID trades some space for safety or speed. RAID 5 gives up one drive's worth of space for parity and survives one failure. RAID 6 gives up two and survives two. Four 4 TB drives give 12 TB in RAID 5 and 8 TB in RAID 6 or RAID 10.
RAID capacity chart
4 TB drives. Search for a drive count.
| Drives | RAID 0 | RAID 5 | RAID 6 | RAID 10 |
|---|---|---|---|---|
| 2 | 8 | 4 | – | – |
| 3 | 12 | 8 | – | – |
| 4 | 16 | 12 | 8 | 8 |
| 5 | 20 | 16 | 12 | – |
| 6 | 24 | 20 | 16 | 12 |
| 8 | 32 | 28 | 24 | 16 |
| 10 | 40 | 36 | 32 | 20 |
| 12 | 48 | 44 | 40 | 24 |
Usable space in TB.
Drives and space
4 TB drives. Slide the number of drives.
Each column is a drive. Blue blocks hold data and orange blocks hold parity.
How to work out RAID space
Level by level. Step through it.
Drive size
Smallest drive.
Rebuilding a large RAID 5 array after a failure takes a long time, during which a second failure loses everything. That is why big arrays use RAID 6.
RAID 5, RAID 6 or RAID 10
Common levels. Compare them.
One parity drive.
Two parity drives.
Mirrored pairs.
Solid-state drives rebuild faster, which makes RAID 5 safer on SSDs than on large hard drives.
Before building
Array checks. Tick each item.
Tick each item as you go.
RAID is not a backup. Keep another copy of anything you cannot lose, preferably off site.
What you get
RAID planning. Each tile explains one result.
TB.
TiB.
Per cent.
Failures.
Read and write.
Drive makers count 1 TB as 10¹² bytes. Computers show it as about 0.91 TiB, so the space looks smaller.
Frequently Asked Questions
No question matches that search.
How do I calculate single parity RAID capacity?
Number of drives minus one, times the size of one drive.
How much space does double parity RAID lose?
Two drives' worth, for double parity.
How many drives does a striped mirror need?
At least four, in an even number.
Which RAID level is best?
RAID 10 for speed and safety, RAID 6 for large arrays of big drives, and RAID 5 for small arrays.
Is RAID a backup?
No. It protects against drive failure, not deletion, ransomware or fire.
Why does my RAID show less space than expected?
Computers show TiB, about 9% smaller than the TB on the box.
Can I mix drive sizes in RAID?
Yes, but every drive only uses as much space as the smallest one.
Space and safety, balanced
Choose the RAID level for the speed and protection you need, and see exactly how much space you keep. Then back it up somewhere else.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.