The usable capacity of a RAID array depends on the RAID level. Even if you use the same number of disks, the amount of storage you can actually use will change depending on whether you choose RAID 0, RAID 1, RAID 5, RAID 6, or RAID 10.
This guide explains the basics of RAID capacity, the formula for each RAID level, practical examples, and important points to keep in mind.
If you care most about capacity, RAID 0 and RAID 5 are attractive. If you care more about safety, RAID 1 and RAID 6 are usually better choices.
In RAID, not all raw disk capacity is always available for storing data. Some RAID levels use part of the total capacity for mirroring or parity. That reduces usable capacity, but improves redundancy and fault tolerance.
For example, if you use four 8TB disks, the usable capacity will be very different depending on the RAID level you choose.
| RAID Level | Usable Capacity Formula | Main Characteristic |
|---|---|---|
| RAID 0 | Number of disks ร Disk size | Maximum capacity, no redundancy |
| RAID 1 | Capacity of one disk | High safety, low capacity efficiency |
| RAID 5 | (Number of disks - 1) ร Disk size | Good balance of capacity and safety |
| RAID 6 | (Number of disks - 2) ร Disk size | Higher safety, less usable space |
| RAID 10 | (Number of disks / 2) ร Disk size | Performance and safety combined |
RAID 0 stripes data across multiple disks. It does not provide redundancy, but it uses all available disk capacity.
Formula: Number of disks ร Disk size
Example: 4 disks ร 8TB = 32TB usable
RAID 1 mirrors the same data across disks. It provides good protection, but usable capacity is limited to the size of one disk.
Formula: Capacity of one disk
Example: 2 disks ร 8TB = 8TB usable
RAID 5 uses the equivalent capacity of one disk for parity. The rest is available for data.
Formula: (Number of disks - 1) ร Disk size
Example: 4 disks ร 8TB = 24TB usable
RAID 6 uses the equivalent capacity of two disks for parity. This reduces usable capacity, but gives you higher fault tolerance.
Formula: (Number of disks - 2) ร Disk size
Example: 4 disks ร 8TB = 16TB usable
RAID 10 combines mirroring and striping. Because it mirrors data, only half of the raw capacity is usable.
Formula: (Number of disks / 2) ร Disk size
Example: 4 disks ร 8TB = 16TB usable
| RAID Level | Usable Capacity |
|---|---|
| RAID 0 | 32TB |
| RAID 1 | 8TB |
| RAID 5 | 24TB |
| RAID 6 | 16TB |
| RAID 10 | 16TB |
Capacity is important, but it should not be the only factor. RAID 0 gives you the most usable space, but offers no protection if a disk fails. RAID 6 gives you less space, but better protection against disk failures.
A better way to choose RAID is to think about what matters more in your setup: maximum storage, stronger safety, or a balance of both.
In most RAID setups, if you mix disks of different sizes, the array will treat them as if they all match the smallest disk.
For example, if you use 8TB, 8TB, 4TB, and 4TB disks together, some of the larger disk capacity may remain unused.
If you want to use space efficiently, matching disk sizes is usually the best approach.
If you do not want to calculate everything manually, you can use the RAID Capacity Calculator to estimate usable capacity in seconds.
Just enter the number of disks, disk size, and RAID level.
If you want to understand how RAID works, the differences between RAID levels, and how to choose the right setup, read this guide too: