Convert the raw disks in a NAS into the usable capacity each RAID level actually leaves you after redundancy.
Each RAID level reserves a different amount of capacity for redundancy: RAID 5 gives up one disk, RAID 6 two, and RAID 10 half. Filesystem metadata and reserved blocks then take a further slice off the formatted total. Buying drives against raw capacity leaves arrays short once parity and formatting are accounted for, and expanding a populated array later is far more disruptive.
NAS RAID Capacity
Usable = data disks × disk size × (1 − filesystem overhead)
Usable = data disks × disk size × (1 − filesystem overhead) Each RAID level reserves a different amount of capacity for redundancy: RAID 5 gives up one disk, RAID 6 two, and RAID 10 half. Filesystem metadata and reserved blocks then take a further slice off the formatted total.
Buying drives against raw capacity leaves arrays short once parity and formatting are accounted for, and expanding a populated array later is far more disruptive.
This calculator takes 4 inputs: Number of disks, Capacity of each disk, RAID level, Filesystem and reserve overhead. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.