RAID & ZFS Storage Calculator
Pick your disk count, size, and layout to see usable capacity, how much goes to parity, and how many drives can fail before you lose data. Runs in your browser.
- Usable (TB)
- 8.0
- Usable (TiB)
- 7.3
- Raw total (TB)
- 16.0
- Efficiency
- 50%
- Disks for data
- 2
- Disks for parity
- 2
- Drives can fail
- 2
- Total drives
- 4
Two parity disks. Survives two failures. The sane default for most NAS builds.
Two things people get wrong
TB is not TiB. Drives are sold in decimal terabytes, but your OS reports binary tebibytes, which are about 9% smaller. A 4 TB drive shows up as roughly 3.6 TiB. The calculator shows both so the second number is the one you will actually see.
RAIDZ1 (RAID 5) is risky on big modern drives. When one disk fails and you rebuild, every other disk gets hammered, and a second failure during that rebuild loses the array. With large drives, RAIDZ2 (two parity disks) is the safer default for most builds.
Still deciding on a parity level? RAID 5 vs RAID 10 covers the tradeoffs. Choosing a NAS OS to run on top? TrueNAS vs Unraid covers ZFS versus a flexible array.