RAID Calculator
Usable capacity, fault tolerance and rebuild exposure for RAID 0, 1, 5, 6, 10, 50 and 60, with the TB against TiB gap made explicit.
Drive labels use decimal. Windows reports binary.
How many RAID sets the stripe spans.
The two numbers that catch people out
The first is the unit. An 8 TB disk holds 8,000,000,000,000 bytes, which Windows and most filesystems report as 7.28 TiB while labelling it "TB". Nothing is missing, the two units just differ by about 9 percent at terabyte scale. Both figures are shown below so you can reconcile a purchase order against what the array actually reports.
The second is what fault tolerance means. RAID 6 survives any two failures. RAID 10 survives one failure per mirror, so with enough bad luck a second failure in the same mirrored pair destroys the array even though other pairs are intact. The figure below gives both the guaranteed and the best-case number where they differ.
Neither is a backup. A parity array protects against a disk dying, not against deletion, ransomware, a controller writing garbage, or the building flooding.