Understanding RAID Capacity and Redundancy

Redundant Array of Independent Disks (RAID) is a technique for combining multiple physical drives into a logical unit for the purposes of redundancy, performance, or both. The fundamental goal is to use drive space more effectively while protecting against drive failures. Each RAID level applies a different scheme of striping and mirroring or parity data so usable space and fault tolerance vary drastically.

The simplest approach is RAID 0 which stripes data across all disks with no redundancy. The formula for usable capacity is purely the sum of all drive sizes:

$$U=ns$$

where n is the number of drives and s is the size of each drive. Because there is no parity or mirroring, RAID 0 offers maximum space but tolerates zero failures. Even a single disk loss destroys the array.

RAID 1 mirrors drives in pairs. For an even number of drives, each pair contains identical data. Usable capacity is therefore:

$$U=\frac{n}{2}s$$

Only half of the raw capacity is usable, but the array can sustain the loss of one drive per mirror set. Odd drive counts waste one disk which becomes a hot spare or unused.

RAID 5 stripes data and single parity across all drives. One drive’s worth of capacity stores parity information, so usable capacity is:

$$U=(n-1)s$$

This level tolerates a single drive failure. When a drive is lost, the array runs in a degraded state until the failed drive is replaced and data is rebuilt from parity. RAID 5 provides a good balance between capacity and redundancy but rebuild times with large disks can be stressful on remaining drives.

RAID 6 extends RAID 5 with double parity. Two drives' worth of capacity are dedicated to parity, yielding:

$$U=(n-2)s$$

RAID 6 can withstand two simultaneous drive failures, making it popular for large arrays. The trade‑off is reduced usable space and additional write overhead.

RAID 10 mirrors striped sets. It requires an even number of drives and effectively mirrors a RAID 0 array. Usable capacity is identical to RAID 1:

$$U=\frac{n}{2}s$$

The performance benefits of striping combine with redundancy, offering fast rebuilds since only the lost mirror pair must be reconstructed. At least two drives can fail provided they are not in the same mirror pair.

Fault Tolerance

The number of drive failures each level can withstand is given by:

RAID Level	Usable Capacity	Drives Tolerated
0	$ns$	0
1	$\frac{n}{2}s$	$\frac{n}{2}$
5	$(n-1)s$	1
6	$(n-2)s$	2
10	$\frac{n}{2}s$	$\frac{n}{2}$

In RAID 10, simultaneous failures are tolerated so long as no mirror pair loses both drives. For example, in a four-drive RAID 10, any two drives may fail as long as they are not the same pair.

Practical Considerations

The above formulas assume all drives are the same size; in practice mixed capacity arrays use the smallest drive as the limiting factor. Hardware controllers and operating systems may also reserve a small amount of space for metadata, slightly reducing usable capacity. When calculating storage for real deployments, always leave headroom for growth and rebuild overhead.

RAID protects against drive failures but is not a substitute for backups. Catastrophic events like power surges, firmware bugs, or user error can still destroy arrays. Maintaining off‑array backups ensures data survival even in worst-case scenarios.

Another practical issue is rebuild time. With today's multi‑terabyte disks, rebuilding a failed RAID 5 or 6 can take many hours or even days, during which the array is vulnerable. RAID 10 rebuilds faster because only the failed mirror needs reconstruction. When planning large arrays, consider using more drives at lower capacity to reduce rebuild windows.

Performance also varies. RAID 0 and RAID 10 offer high throughput because they stripe data, while RAID 5 and 6 incur parity computation costs. Solid State Drives (SSDs) can mitigate some of these penalties but also complicate parity wear. Many modern systems use hardware controllers or software like ZFS and mdadm to handle these details.

Finally, note that RAID levels above 6, such as RAID 50, 60, or proprietary schemes like SHR, follow similar principles but combine multiple arrays. This calculator focuses on core levels that cover most consumer and small business scenarios.

Whether you are building a home NAS or provisioning enterprise storage, understanding how each RAID level uses drive capacity is essential. Carefully weighing the trade‑offs between usable space, performance, and redundancy ensures the array fits your needs and budget.