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Free planning tool

RAID Capacity Calculator

See how much storage survives parity, mirrors, decimal-to-binary conversion, and healthy free-space headroom before you buy drives.

Array layout

RAID 6 reserves two drives for parity and can survive two drive failures.

Capacity result

Raw capacity
48.00 TB
Usable array capacity
24.00 TB

21.83 TiB as most operating systems report it.

Recommended working limit
20.40 TB

After the selected free-space reserve.

Array overhead24.00 TB

Storage efficiency50%

Failure tolerance2 drives

RAID is not a backup. It improves availability after a drive failure, but it does not protect against deletion, corruption, theft, fire, or a failed controller.

Why a 12 TB drive does not look like 12 TB

Drive makers label capacity in decimal terabytes: one TB is 1,000,000,000,000 bytes. Many operating systems display binary tebibytes, where one TiB is 1,099,511,627,776 bytes. A 12 TB drive therefore appears as about 10.91 TiB before RAID overhead or formatting.

How each RAID level uses drives

RAID 0 uses every drive for data and provides no redundancy. RAID 1 keeps one drive's worth of usable capacity across a mirror. RAID 5 and RAID 6 reserve the equivalent of one or two drives for parity. RAID 10 requires an even drive count and uses half of the raw capacity for mirrored pairs.

Leave working space

A full array is harder to manage and can perform poorly depending on the filesystem and workload. The working limit applies a planning reserve after RAID overhead. It is not filesystem metadata overhead, snapshot space, hot-spare capacity, or backup capacity, so budget those separately.

Mixed drive sizes and hot spares

This calculator assumes every active drive has the same capacity. Conventional RAID usually treats each member as though it were the size of the smallest drive. Do not include a dedicated hot spare in the drive count because it contributes no usable capacity until it replaces a failed member.

Building the rest of the system? Start with our home server build guide, compare mini PCs for home servers, or size a recording workload with the security camera storage calculator.