RAID Calculator

How to Use the RAID Calculator

1. Pick the RAID level: 0, 1, 5, 6 or 10.
2. Choose identical drives, or mixed sizes to apply the smallest-drive rule.
3. Enter how many drives you have and the size printed on each one.
4. Read the usable capacity in TB and TiB, then compare every level in the table below.

Choose a RAID level, then describe your drives. With Identical drives, enter how many drives you have and the size printed on each one. With Mixed sizes, list every drive, such as 4, 4, 6, 8. Pick TB or GB to match the label on the drives. The panel shows the usable capacity in decimal TB and in binary TiB, a bar that splits the raw space into usable, redundancy and unused parts, the number of drive failures the array can survive, and theoretical read and write speed compared with a single drive.

The table at the bottom of the result panel repeats the calculation for every level with the same drives, so you can compare RAID 5 with RAID 6 or RAID 10 at a glance. Tap a row to switch to that level.

RAID Capacity Formulas

With n drives and S as the size of the smallest drive, usable capacity is:

The write cost is the classic write penalty: a small random write on RAID 5 reads the old data and parity, then writes new data and parity. Sequential writes on parity arrays avoid most of it.

Worked Example

Four 4 TB drives in RAID 5 give (4 − 1) × 4 TB = 12 TB usable out of 16 TB raw, a 75% efficiency, and the array survives one failed drive. In binary units that is 12 × 10¹² ÷ 2⁴⁰ = 10.91 TiB, which Windows shows as 10.9 TB. The same drives in RAID 6 or RAID 10 give 8 TB, survive two failures in RAID 6, and survive one guaranteed failure in RAID 10.

With mixed drives of 4, 4, 6 and 8 TB in RAID 5, the smallest drive sets the size of every member: (4 − 1) × 4 TB = 12 TB usable from 22 TB raw. The extra 2 TB and 4 TB on the larger drives, 6 TB in total, stay unused by the array.

TB Versus TiB

Drive makers count in decimal units, so 1 TB is 1,000,000,000,000 bytes. Many operating systems count in binary units, where 1 TiB is 2⁴⁰ = 1,099,511,627,776 bytes, and Windows labels that unit TB. The gap is about 9% at the terabyte scale, which is why a new array always looks smaller than expected. The NIST page on binary prefixes explains the official names. File system metadata and reserved space reduce the final free space a little further.

Limits and Practical Advice

• RAID is not a backup. It keeps a server running through a drive failure, but deleted files, ransomware and controller faults affect every copy. Keep separate backups.
• Speed figures are theoretical. Real results depend on the controller, caching, file system and workload.
• Rebuilds take time. A parity rebuild reads every surviving drive. On large drives this can take many hours, so RAID 6 or RAID 10 is safer than RAID 5 for big arrays.
• Hot spares sit outside the array, so do not count them as drives here.
• Systems such as ZFS, Btrfs and some NAS hybrid modes can use mixed sizes more efficiently than classic RAID. This calculator follows the standard smallest-drive rule.

For network storage planning, the SNIA dictionary defines each RAID term used here.

Frequently asked questions