Technology

RAID Calculator

Calculate usable storage capacity, raw capacity, fault tolerance, and efficiency for RAID 0, 1, 5, 6, and 10 arrays from your disk count and size.

RAID Calculator

Number of disks

Single disk size

Unit

RAID level

Method

How this calculator works

Raw = n × disk size. Usable: RAID 0 = n × size; RAID 1 = size; RAID 5 = (n − 1) × size; RAID 6 = (n − 2) × size; RAID 10 = (n ÷ 2) × size. Efficiency = usable ÷ raw × 100%.

Enter the number of disks in your array.
Enter the capacity of a single disk and choose GB or TB (all disks are assumed identical).
Select the RAID level you plan to use.
The tool checks the minimum disk requirement for that level and reports usable capacity, raw capacity, fault tolerance, and efficiency.

Examples

Worked examples

Real numbers, end-to-end results.

4 × 4 TB disks in RAID 5

Usable = 12 TB, Raw = 16 TB, Efficiency 75%

RAID 5 usable = (n − 1) × size = 3 × 4 TB = 12 TB, tolerating one disk failure.

6 × 2 TB disks in RAID 6

Usable = 8 TB, Raw = 12 TB, Efficiency 66.7%

RAID 6 usable = (n − 2) × size = 4 × 2 TB = 8 TB, tolerating two disk failures.

Use cases

When to use it

Planning a NAS or home server build before buying drives.
Comparing the space cost of redundancy across RAID levels.
Sizing an array to meet a target usable capacity.
Explaining fault-tolerance trade-offs to a team or client.

FAQ

Frequently asked questions

What does a RAID calculator tell me?

It converts a set of identical disks into the real numbers that matter for an array: how much usable space you get, how much raw capacity you started with, how many disks can fail without data loss, and what percentage of your raw storage is actually available for data.

Which RAID level gives the most usable space?

RAID 0 gives 100% usable capacity because it stripes data across all disks with no redundancy—but a single disk failure destroys the entire array. Among redundant levels, RAID 5 (one parity disk) is the most space-efficient, followed by RAID 6 (two parity disks) and then mirrored levels like RAID 1 and RAID 10.

How is usable capacity calculated for RAID 5 and RAID 6?

RAID 5 reserves the equivalent of one disk for parity, so usable capacity is (n − 1) × disk size. RAID 6 reserves two disks' worth of parity, giving (n − 2) × disk size. The parity is distributed across all disks, but the space cost equals one or two full disks respectively.

What is fault tolerance?

Fault tolerance is the number of disks that can fail simultaneously while the array still keeps all your data. RAID 0 tolerates zero failures, RAID 5 tolerates one, RAID 6 tolerates two, RAID 1 can survive losing all but one mirror, and RAID 10 always survives one failure and up to one per mirrored pair in the best case.

Does RAID replace backups?

No. RAID protects against disk hardware failure and improves uptime, but it does not guard against accidental deletion, ransomware, file corruption, or the loss of the whole machine. Always keep separate, ideally off-site, backups in addition to any RAID array.