RAID Calculator - Capacity and failure simulation
Compare RAID 0, 1, 5, 6 and 10 by usable capacity, then click disks to simulate failures.
Compare RAID levels by usable capacity
- Type how many active disks the array will use (1 to 32) and the capacity of each disk — for example 4 disks of 2 TB. Hot spares stay in a separate field so they never inflate the usable total.
- Worked example: 4 x 2 TB gives 8 TB on RAID 0, 2 TB on RAID 1 (one complete mirror), 6 TB on RAID 5 and 4 TB on both RAID 6 and RAID 10. These follow directly from (N - 1) x S and (N - 2) x S with N = 4 and S = 2 TB.
- Open the individual disk sizes when your disks differ: the conventional simulation caps every disk at the smallest one, so 2 + 2 + 4 + 4 TB behaves like 4 x 2 TB with 4 TB left unused.
- Pick a level to simulate, then click disks to fail them. RAID 10 treats pairs honestly: two failures inside the same mirror keep the array degraded, while losing both members of any pair stops it.
- Read TB as decimal terabytes and TiB as binary tebibytes: 2 TB formats to about 1.82 TiB, and the operating system plus filesystem metadata take a further cut, so treat every figure as a theoretical maximum.
- Copy the text summary or a link that reopens the same setup. Everything runs locally — no disk inventory ever leaves your browser.
RAID capacity questions
Why does RAID 1 with four disks still show 2 TB?
This page models RAID 1 as one complete mirror across all N disks, which yields exactly one disk of usable space (S). It is not modelled as independent pairs — that layout is RAID 10, which with 4 x 2 TB gives 4 TB instead.
Why do mixed disks waste space?
Conventional RAID stripes data evenly, so every disk can only contribute up to the smallest one. With 2 + 2 + 4 + 4 TB, each disk counts as 2 TB and 4 TB stays unused. Some vendor-specific layouts reuse that extra space, which is why the page labels the result conventional.
What is the difference between TB and TiB?
TB is decimal (1 TB = 1,000,000,000,000 bytes) and TiB is binary (1 TiB = 1,099,511,627,776 bytes), so the same array reads smaller in TiB: 8 TB is about 7.28 TiB. Drive makers advertise TB while operating systems usually display TiB.
Why are there no speed or rebuild-time forecasts?
Throughput and rebuild duration depend on the controller, drive speeds, workload and array state — none of which a capacity formula can see. Any page that prints MB/s or hours from disk sizes alone is inventing numbers, so this one refuses to.
Does RAID replace backups, and what do hot spares do?
No: RAID survives disk failures but not deletion, ransomware, controller faults, fire or theft. A hot spare is a standby disk that can start rebuilding automatically, but it is not instant tolerance — during the rebuild the array is still exposed, which is why the simulator keeps spares aside.