RAID Calculator
Estimate usable storage, reserves, overhead, and fault tolerance
How the active drives are combined.
Total drives installed in the array.
Capacity printed for each identical drive.
Use decimal for drive labels or binary for OS-style units.
Installed drives reserved for rebuilds.
Space held back for filesystem health.
Optional planning space for snapshots.
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How RAID Capacity Works
Installed drive capacity is not the same as available storage.
Raw versus usable
Raw capacity counts active drives before RAID overhead. Usable capacity is what remains after parity, mirroring, or RAID 10 pairs.
Hot spares are standby drives
A hot spare can help an array start rebuilding sooner, but it does not add usable capacity while it is reserved as a spare.
Reserve space keeps plans realistic
Filesystems, snapshots, and copy-on-write pools often need free space. A planning reserve keeps the final estimate closer to use.
RAID Capacity Examples
Common storage planning scenarios for home labs and NAS builds.
One drive worth of capacity is used for single-parity protection.
Five active drives remain after the spare, with two drives of parity.
Half the active drives provide capacity and half provide mirrors.
RAID Calculator FAQ
Answers about usable capacity, hot spares, reserves, TB versus TiB, and backups.
What does a RAID calculator estimate?
It estimates how much storage remains after RAID parity or mirroring, hot spares, and optional filesystem or snapshot reserves.
Why is usable capacity lower than raw drive capacity?
RAID levels spend some capacity on parity, mirrors, or unused mirror-pair drives. Filesystems and snapshot plans can reserve additional space.
What is the difference between TB and TiB?
TB is decimal storage used by drive manufacturers. TiB is binary storage often reported by operating systems, so the same disk can look smaller in TiB.
Does RAID replace backups?
No. RAID can help with drive failure tolerance, but it does not protect against deletion, corruption, malware, theft, or site loss.
Why does RAID 10 waste one drive with an odd count?
RAID 10 is built from mirror pairs. With an odd number of active drives, one drive cannot form a complete pair in this simplified planning model.