What Is a RAID Calculator?

A RAID calculator turns a set of drives and a RAID level into the numbers that decide whether an array is worth building: usable capacity, how many drives can fail before data is lost, effective random IOPS after the write penalty, rebuild time, and the odds of an unrecoverable read error during that rebuild. It is the same arithmetic a storage administrator does before buying drives, just collected in one place so the trade-offs are visible at a glance rather than discovered two years into production.

The reason it is worth calculating rather than eyeballing is that the trade-offs move in opposite directions. Striping gives the best capacity and speed but no redundancy at all. Mirroring protects data at the cost of half the raw capacity. Parity levels recover the capacity but pay for it in writes and in a long, fragile rebuild window. Capacity alone is a poor guide: choosing RAID 0 on a backup target can destroy the backups it was meant to hold.

How It Works

Usable capacity is a per-level formula applied to the active drive count. RAID 0 multiplies drive count by capacity; RAID 1 mirrors and exposes only one drive's worth; RAID 5 reserves one drive for distributed parity; RAID 6 reserves two; RAID 10 pairs drives and then stripes across the pairs, so it exposes half the raw capacity and needs an even drive count. Hot spares come out of the active count before any of this, and the filesystem reservation is then deducted from what remains, which is why ZFS pools look smaller than the arithmetic on paper.

Performance follows from the write penalty. A random stripe write on RAID 5 costs two reads and two writes, on RAID 6 three and three, while mirroring costs two writes and striping costs one. Effective write IOPS is therefore the sum of the active drives' IOPS divided by that penalty, while read IOPS simply scale with the number of active drives. Rebuild time is the capacity of one drive divided by the throughput the controller can spare, so an 8 TB drive at 180 MB/s takes about 12 hours.

Reliability is where the calculator earns its keep. The expected number of unrecoverable read errors during a rebuild is the number of bytes the rebuild must read divided by the drive's rated error interval, and the probability of at least one error is one minus the exponential of that figure. On enterprise drives rated 1 in 10 to the 15th bits, an 8 TB member carries roughly a 6 percent chance; on desktop drives rated a decade lower, the same rebuild faces roughly a 47 percent chance. On RAID 5 any such error is fatal, because there is no second parity block to reconstruct the missing stripe.

Understanding Your Results

The headline is usable capacity in decimal terabytes with the binary figure underneath, since the operating system reports TiB. The three-stat bar carries the three practical decision points: fault tolerance in drives, effective write IOPS, and rebuild time. The breakdown table then shows the raw-to-usable bridge, the stripe width, the write penalty, and the two risk figures that most array builders ignore.

Use the risk rows actively. If the rebuild probability is above 10 percent and the array has only single-fault tolerance, RAID 6 or a mirrored layout is the safer choice at that drive size, and adding a hot spare converts a failed drive into a background rebuild rather than a manual intervention. Fleet reporting has put large-scale hard drive failure rates near 1 percent a year, so on an eight-drive array the annual chance that at least one drive fails is genuinely around 8 percent, and the array will spend meaningful time degraded at some point in its service life.