RAID Calculator
Enter the number and size of your disks to compare usable capacity, how many disks can fail, and the performance profile of each RAID level.
| Level | Usable capacity | Shown in OS | Efficiency | Disks that can fail | Profile |
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All disks are assumed to be the same size. Figures are raw capacity after RAID, before file system formatting.
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RAID levels at a glance
- RAID 0 (striping): data is split across all disks. Fastest, with all capacity usable, but a single failed disk means all data is lost.
- RAID 1 (mirroring): every disk holds an identical copy. Safe and simple, but capacity equals a single disk.
- RAID 5: data and parity are spread across all disks, minimum 3 disks. Survives one failed disk and gives up one disk of capacity.
- RAID 6: like RAID 5 with double parity, minimum 4 disks. Survives two failed disks, safer for large-capacity disks.
- RAID 10 (1+0): a stripe of mirrors, minimum 4 disks in an even number. The best write performance among the redundant levels.
Tips for choosing
With large disks (8 TB and up), a RAID 5 rebuild can take a very long time, and the risk of a second disk failing during the rebuild goes up. For large capacities, RAID 6 or RAID 10 is recommended. Whatever you choose, keep a separate backup.
Frequently asked questions
Which RAID level is best for a server?
For databases and write-heavy applications, RAID 10 usually offers the best balance of performance and safety. RAID 6 suits large storage such as file servers or backup targets because it survives two failed disks. RAID 5 is capacity-efficient, but risky with large disks because rebuilds take a long time.
Is RAID the same as a backup?
No. RAID protects against disk failure, but not against deleted files, ransomware, controller failure, or a disaster at the site. Important data still needs a separate backup, ideally in a different location.
Why does the operating system show less capacity?
Disk manufacturers use decimal units (1 TB = 1,000,000,000,000 bytes), while many operating systems use binary units (1 TiB = 1,099,511,627,776 bytes). That is why a 4 TB disk shows up as about 3.64 TiB. This calculator shows both.
Can I mix disks of different sizes?
Yes, but in standard RAID each disk is only used up to the size of the smallest disk in the array. This calculator assumes all disks are the same size.
How many disks can fail in RAID 10?
At least one disk is guaranteed safe. RAID 10 can survive up to half of its disks failing, as long as no two disks from the same mirror pair fail at the same time.
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